Light emitting device and display device including the same

ABSTRACT

A light emitting device, a method of manufacturing the same, and a display device including the same are disclosed. The light emitting device including a first electrode and a second electrode facing each other, an emission layer disposed between the first electrode and the second electrode, the emission layer including quantum dots, and a charge auxiliary layer disposed between the emission layer and the second electrode, wherein the emission layer includes a first surface facing the charge auxiliary layer and an opposite second surface, the quantum dots include a first organic ligand on a surface of the quantum dots, in the emission layer, an amount of the first organic ligand in a portion adjacent to the first surface is larger than an amount of the first organic ligand in a portion adjacent to the second surface.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of application Ser. No.16/549,472, filed Aug. 23, 2019, which claims priority to and thebenefit of Korean Patent Application No. 10-2018-0099520 filed in theKorean Intellectual Property Office on Aug. 24, 2018, all the benefitsaccruing therefrom under 35 U.S.C. § 119, the content of which in itsentirety is herein incorporated by reference.

BACKGROUND 1. Field

A light emitting device and a display device are disclosed.

2. Description of the Related Art

Physical characteristics (e.g., energy bandgaps, melting points, etc.)of nanoparticles that are intrinsic characteristics may be controlled bychanging the particle sizes of the nanoparticles, unlike bulk materials.For example, when being are supplied with photoenergy or electricenergy, semiconductor nanocrystals (also referred to as quantum dots)may emit light in a wavelength corresponding to sizes of the quantumdots. Accordingly, the quantum dots may be used as a light emittingelement emitting light of a predetermined wavelength.

SUMMARY

A light emitting device may include quantum dots as a light emittingelement. Improvement in performance of light emitting devices includingquantum dots is desired.

An embodiment provides a light emitting device capable of realizingimproved performance.

An embodiment provides a display device including the light emittingdevice.

A light emitting device according to an embodiment includes

a first electrode and a second electrode facing each other,

an emission layer disposed between the first electrode and the secondelectrode, the emission layer including quantum dots, and

a charge auxiliary layer disposed between the emission layer and thesecond electrode,

wherein the emission layer includes a first surface facing the chargeauxiliary layer and an opposite second surface,

the quantum dots include a first organic ligand (e.g., bound on asurface thereof), and

in the emission layer, an amount (e.g., a concentration) of the firstorganic ligand in a portion adjacent to the first surface is larger thanan amount of the first organic ligand in a portion adjacent to thesecond surface.

The amount of the first organic ligand in the portion adjacent to thefirst surface may be at least about 20% larger than the amount of thefirst organic ligand in the portion adjacent to the second surface.

The amount of the first organic ligand in the portion adjacent to thefirst surface may be at least about 30% larger than the amount of thefirst organic ligand in the portion adjacent to the second surface.

The charge auxiliary layer may include nanoparticles including zincmetal oxide.

The zinc metal oxide may be represented by Chemical Formula 1:

Zn_(1-x)M_(x)O  Chemical Formula 1

In Chemical Formula 1,

M is Mg, Ca, Zr, W, Li, Ti, Y, Al, or a combination thereof, and

0≤x≤0.5.

The metal oxide may include zinc oxide, zinc magnesium oxide, or acombination thereof.

An average particle size of the nanoparticles may be greater than orequal to about 1 nanometer (nm).

An average particle size of the nanoparticles may be less than or equalto about 10 nm.

A work function of the first electrode may be greater than a workfunction of the second electrode.

The first electrode may include indium tin oxide.

The second electrode may include a conductive metal.

The quantum dots may not include cadmium.

The quantum dots may include indium and phosphorus.

The quantum dots may include a chalcogen element and zinc.

The quantum dots may be configured to emit light having a same color.

In the emission layer, the portion adjacent to the second surface mayfurther include a halogen.

The halogen may include fluorine, chlorine, bromine, iodine, or acombination thereof.

The halogen may be chlorine.

In the emission layer, the portion adjacent to the first surface (e.g.,a first emission layer) may not include chlorine.

In the emission layer, the portion adjacent to the first surface (e.g.,a first emission layer) may further include chlorine.

In the emission layer, an amount of a halogen (e.g., chlorine) in theportion adjacent to the second surface (e.g., a second emission layer)may be greater than an amount of a halogen (e.g., chlorine) in theportion adjacent to the first surface (e.g., a first emission layer).

A hole mobility in the portion adjacent to the first surface may be lessthan a hole mobility in the portion adjacent to the second surface. Ahole mobility in the portion adjacent to the second surface may begreater than a hole mobility in the portion adjacent to the firstsurface.

The first organic ligand may include RCOOH, RNH₂, R₂NH, R₃N, RSH, R₃PO,R₃P, ROH, RCOOR, RPO(OH)₂, RHPOOH, RHPOOH, or a combination thereof,wherein, each R is independently a C3 to C40 substituted orunsubstituted aliphatic hydrocarbon group, a C6 to C40 substituted orunsubstituted aromatic hydrocarbon group, or a combination thereof.

The first organic ligand may not include a thiol organic ligand.

The thiol organic ligand may include RSH wherein, R is independently aC3 to C40 substituted or unsubstituted aliphatic hydrocarbon group, a C6to C40 substituted or unsubstituted aromatic hydrocarbon group, or acombination thereof.

A thickness of the emission layer may be greater than or equal to about2 nm.

A thickness of the emission layer may be greater than or equal to about20 nm.

A thickness of the emission layer may be less than or equal to about 100nm.

The emission layer may include a first emission layer including thefirst surface and a second emission layer including the second surface.

Each of the first emission layer and the second emission layer mayinclude the first organic ligand.

A thickness of the first emission layer may be greater than or equal toabout 3 nm.

A thickness of the first emission layer may be less than or equal toabout 50 nm.

A thickness of the second emission layer may be greater than or equal toabout 3 nm.

A thickness of the second emission layer may be less than or equal toabout 50 nm.

The first emission layer may not include a halogen.

The second emission layer may further include a halogen (e.g.,chlorine).

The first emission layer may further include chlorine.

An amount of chlorine in the second emission layer may be greater thanan amount of chlorine in the first emission layer.

A hole mobility in the second emission layer may be greater than a holemobility in the first emission layer.

The hole mobility in the second emission layer may be at least 1.5 timesof the hole mobility in the first emission layer.

The hole mobility in the second emission layer may be at least 2 timesof the hole mobility in the first emission layer.

The second emission layer may be insoluble to a C1 to C10 alcoholsolvent, cyclohexyl acetate, acetone, toluene, hexane, cyclohexane, a C1to C10 alkane solvent, or a combination thereof.

The first emission layer may be insoluble to a C1 to C10 alcoholsolvent. The second emission layer may not include an arylamine, a thiolcompound, or a combination thereof.

The second emission layer may further include a second organic ligandthat is different from the first organic ligand. The second organicligand may include a C3 to C20 organic compound including a thiol groupand an alcohol group.

The first emission layer may not include an organic compound including aheterocycle including oxygen, sulfur, nitrogen, or silicon.

The first emission layer may further include a second organic ligandthat is different from the first organic ligand and the second organicligand may further include C3 to C40 alkane thiol.

The first emission layer and/or the second emission layer may notinclude a thiol organic ligand.

In an embodiment, a method of manufacturing the aforementioned lightemitting device includes,

forming the emission layer on the first electrode; forming the chargeauxiliary layer on the emission layer; and forming the second electrodeon the charge auxiliary layer,

wherein the forming of the emission layer includes

forming a first layer including a plurality of quantum dots having afirst organic ligand on the surface;

removing the ligand from the first layer to remove at least a portion ofthe first organic ligand; and

forming a second layer including the quantum dots including the firstorganic ligand on the surface of the quantum dots;

removing at least a portion (or a portion of) of the first organicligand from the first layer to form a treated first layer; and

forming a second layer including the quantum dots including the firstorganic ligand on the surface of the quantum dots on the treated firstlayer to provide the light emitting device, wherein the second layercomprises the amount of the first organic ligand larger than the amountof the first organic ligand in the treated first layer.

Removing at least a portion of the first organic ligand from the firstlayer may include,

preparing an alcohol solution of a metal halide;

contacting the alcohol solution with the first layer; and

removing the alcohol solution from the first layer and drying the firstlayer.

The metal halide may include a zinc halide.

The metal halide may include a fluoride, a chloride, a bromide, aniodide, or a combination thereof.

Removing at least a portion of the first organic ligand from the firstlayer may including removing greater than or equal to about 20 weightpercent of the first organic ligand from the first layer, based on atotal weight of the first organic ligand in the first layer.

An embodiment provides a display device including the aforementionedlight emitting device.

In an embodiment, a method of manufacturing a light emitting deviceincludes:

obtaining an emission layer, wherein the emission layer comprisesquantum dots comprising a first organic ligand;

changing an amount of the first organic ligand on at least one surfaceof the emission layer;

contacting a charge auxiliary layer with a first surface of the emissionlayer;

contacting a second electrode with the charge auxiliary layer; and

contacting a second surface of the emission layer with a firstelectrode,

wherein the second surface of the emission layer is opposite the firstsurface of the emission layer,

wherein an amount of the organic ligand in a portion adjacent to thefirst surface of the emission layer is larger than an amount of theorganic ligand in a portion adjacent to the second surface of theemission layer, and

wherein a thickness of the portion of the emission layer adjacent to thefirst surface of the emission layer is equal to a thickness of theportion of the emission layer adjacent to the second surface of theemission layer.

According to an embodiment, an electroluminescent device having improvedefficiency and life-span simultaneously improve may be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages and features of this disclosure willbecome more apparent by describing in further detail exemplaryembodiments thereof with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic cross-sectional view of a quantum dot (QD) lightemitting diode (LED) device according to an embodiment.

FIG. 2 is a schematic cross-sectional view of a QD LED device accordingto an embodiment.

FIG. 3 is a schematic cross-sectional view of a QD LED device accordingto an embodiment.

FIG. 4 shows infrared spectroscopy results of the quantum dot emissionlayers produced in Reference Example 3-1 and Reference Example 3-2.

FIG. 5 shows infrared spectroscopy results of the quantum dot emissionlayers produced in Reference Example 4-1 and Reference Example 4-2.

FIG. 6 is a graph of External Quantum Efficiency (EQE) (percent (%))versus Luminance (candelas per square meter (Cd/m²)) showingelectroluminescence properties of the electroluminescent devices ofExample 1 and Comparative Examples 1 and 2.

FIG. 7 is a graph of logarithm of current density (J) (milliamperes persquare centimeter (mA/cm²)) (log J) versus Voltage (volts (V))) showingelectroluminescence properties of the electroluminescent devices ofExample 1 and Comparative Examples 1 and 2.

FIG. 8 is a view showing a process of Experimental Example 1 and resultsthereof.

DETAILED DESCRIPTION

Hereinafter, example embodiments of the present disclosure will bedescribed in detail so that a person skilled in the art would understandthe same. This disclosure may, however, be embodied in many differentforms and is not construed as limited to the example embodiments setforth herein.

In the drawings, the thickness of layers, films, panels, regions, etc.,are exaggerated for clarity. Like reference numerals designate likeelements throughout the specification. It will be understood that whenan element such as a layer, film, region, or substrate is referred to asbeing “on” another element, it can be directly on the other element orintervening elements may also be present. In contrast, when an elementis referred to as being “directly on” another element, there are nointervening elements present.

It will be understood that, although the terms “first,” “second,”“third” etc. may be used herein to describe various elements,components, regions, layers and/or sections, these elements, components,regions, layers and/or sections should not be limited by these terms.These terms are only used to distinguish one element, component, region,layer or section from another element, component, region, layer orsection. Thus, “a first element,” “component,” “region,” “layer,” or“section” discussed below could be termed a second element, component,region, layer, or section without departing from the teachings herein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms, including “at least one,” unless the content clearly indicatesotherwise. “At least one” is not to be construed as limiting “a” or“an.” “or” means “and/or.” As used herein, the term “and/or” includesany and all combinations of one or more of the associated listed items.It will be further understood that the terms “comprises” and/or“comprising,” or “includes” and/or “including” when used in thisspecification, specify the presence of stated features, regions,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,regions, integers, steps, operations, elements, components, and/orgroups thereof.

Furthermore, relative terms, such as “upper,” may be used herein todescribe one element's relationship to another element as illustrated inthe Figures. It will be understood that relative terms are intended toencompass different orientations of the device in addition to theorientation depicted in the Figures. For example, if the device in oneof the figures is turned over, elements described as being on the“lower” side of other elements would then be oriented on “upper” sidesof the other elements. The exemplary term “upper,” can therefore,encompasses both an orientation of “lower” and “upper,” depending on theparticular orientation of the figure.

“About” as used herein is inclusive of the stated value and means withinan acceptable range of deviation for the particular value as determinedby one of ordinary skill in the art, considering the measurement inquestion and the error associated with measurement of the particularquantity (i.e., the limitations of the measurement system). For example,“about” can mean within one or more standard deviations, or within ±10%,or 5% of the stated value.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

Exemplary embodiments are described herein with reference to crosssection illustrations that are schematic illustrations of idealizedembodiments. As such, variations from the shapes of the illustrations asa result, for example, of manufacturing techniques and/or tolerances,are to be expected. Thus, embodiments described herein should not beconstrued as limited to the particular shapes of regions as illustratedherein but are to include deviations in shapes that result, for example,from manufacturing. For example, a region illustrated or described asflat may, typically, have rough and/or nonlinear features. Moreover,sharp angles that are illustrated may be rounded. Thus, the regionsillustrated in the figures are schematic in nature and their shapes arenot intended to illustrate the precise shape of a region and are notintended to limit the scope of the present claims.

Hereinafter, a work function or energy level (e.g., a highest occupiedmolecular orbital (HOMO) energy level or lowest unoccupied molecularorbital (LUMO) energy level) is expressed as an absolute value from avacuum level. In addition, when the work function or the energy level isreferred to be “deep,” “high” or “large,” the work function or theenergy level has a large absolute value based on “0 eV” of the vacuumlevel, while when the work function or the energy level is referred tobe “shallow,” “low,” or “small,” the work function or energy level has asmall absolute value based on “0 eV” of the vacuum level.

As used herein, “Group” may refer to a group of Periodic Table.

As used herein, “Group I” may refer to Group IA and Group IB, andexamples thereof may include Li, Na, K, Rb, and Cs, but are not limitedthereto.

As used herein, “Group II” may refer to Group IIA and Group IIB, andexamples of a Group II metal may be Cd, Zn, Hg, and Mg, but are notlimited thereto.

As used herein, “Group III” may refer to Group IIIA and Group IIIB, andexamples of a Group III metal may be Al, In, Ga, and TI, but are notlimited thereto.

As used herein, “Group IV” may refer to Group IVA and Group IVB, andexamples of a Group IV metal may be Si, Ge, and Sn, but are not limitedthereto. As used herein, the term “metal” may include a semi-metal suchas Si.

As used herein, “Group V” may refer to Group VA, and examples thereofmay include nitrogen, phosphorus, arsenic, antimony, and bismuth, butare not limited thereto.

As used herein, “Group VI” may refer to Group VIA, and examples thereofmay include sulfur, selenium, and tellurium, but are not limitedthereto.

As used herein, when a definition is not otherwise provided,“substituted” refers to replacement of hydrogen of a compound, a group,or a moiety by a C1 to C30 alkyl group, a C2 to C30 alkenyl group, a C2to C30 alkynyl group, a C2 to C30 epoxy group, a C2 to C30 alkenylgroup, a C2 to C30 alkylester group, a C3 to C30 alkenylester group(e.g., acrylate group, methacrylate group), a C6 to C30 aryl group, a C7to C30 alkylaryl group, a C1 to C30 alkoxy group, a C1 to C30heteroalkyl group, a C3 to C40 heteroaryl group, a C3 to C30heteroalkylaryl group, a C3 to C30 cycloalkyl group, a C3 to C15cycloalkenyl group, a C6 to C30 cycloalkynyl group, a C2 to C30heterocycloalkyl group, a halogen (—F, —Cl, —Br, or —I), a hydroxy group(—OH), a nitro group (—NO₂), a thiocyanate group (—SCN), a cyano group(—CN), an amino group (—NRR′ wherein R and R′ are independently hydrogenor a C1 to C6 alkyl group), an azido group (—N₃), an amidino group(—O(═NH)NH₂), a hydrazino group (—NHNH₂), a hydrazono group (═N(NH₂)),an aldehyde group (—C(═O)H), a carbamoyl group (—C(O)NH₂), a thiol group(—SH), an ester group (—O(═O)OR, wherein R is a C1 to C6 alkyl group ora C6 to C12 aryl group), a carboxyl group (—COOH) or a salt thereof(—O(═O)OM, wherein M is an organic or inorganic cation), a sulfonic acidgroup (—SO₃H) or a salt thereof (—SO₃M, wherein M is an organic orinorganic cation), a phosphoric acid group (—PO₃H₂) or a salt thereof(—PO₃MH or —PO₃M₂, wherein M is an organic or inorganic cation), or acombination thereof.

As used herein, when a definition is not otherwise provided, ahydrocarbon group refers to a group including carbon and hydrogen (e.g.,alkyl, alkenyl, alkynyl, aryl group, etc.). The hydrocarbon group may bea group having a monovalence or greater formed by removal of one or morehydrogen atoms from, alkane, alkene, alkyne, or arene. In thehydrocarbon group, at least one methylene may be replaced by an oxidemoiety, a carbonyl moiety, an ester moiety, —NH—, or a combinationthereof.

As used herein, when a definition is not otherwise provided, “alkyl”refers to a linear or branched saturated monovalent hydrocarbon group(methyl, ethyl hexyl, etc.).

As used herein, when a definition is not otherwise provided, “alkenyl”refers to a linear or branched monovalent hydrocarbon group having oneor more carbon-carbon double bond.

As used herein, when a definition is not otherwise provided, “alkynyl”refers to a linear or branched monovalent hydrocarbon group having oneor more carbon-carbon triple bond.

As used herein, when a definition is not otherwise provided, “aryl”refers to a group formed by removal of at least one hydrogen from anaromatic group (e.g., phenyl or naphthyl group).

As used herein, when a definition is not otherwise provided, “hetero”refers to one including 1 to 3 heteroatoms of N, O, S, Si, P, or acombination thereof.

As used herein, when a definition is not otherwise provided,“heteroaryl” refers to an aromatic group that comprises at least oneheteroatom covalently bonded to one or more carbon atoms of aromaticring.

As used herein, an average size of particles (or quantum dots) may bedetermined by using an electron microscope analysis and optionally acommercially available image analysis program (Image J). The average maybe mean or median.

FIG. 1 is a schematic cross-sectional view of a light emitting deviceaccording to an embodiment.

Referring to FIG. 1, a light emitting device 10 according to anembodiment includes a first electrode 11 and a second electrode 15facing each other, an emission layer 13 disposed between the firstelectrode 11 and the second electrode 15 and including quantum dots, anda charge auxiliary layer disposed between the emission layer and thesecond electrode, wherein the emission layer 13 includes a first surfacefacing the charge auxiliary layer and an opposite second surface, thequantum dots include a first organic ligand on the surface, and in theemission layer, an amount (or a concentration), e.g., a total weight ora number of molecules, of the organic ligand at a portion adjacent tothe first surface is larger, e.g., greater, than an amount, e.g., atotal weight or a number of molecules, of the organic ligand at aportion adjacent to the second surface.

A thickness of the portion of the emission layer adjacent to the firstsurface may be equal to (or different from) a thickness of the portionof the emission layer adjacent to the second surface. A thickness of theportion of the emission layer adjacent to the first surface may be lessthan or equal to a thickness of the portion of the emission layeradjacent to the second surface. A thickness of the portion of theemission layer adjacent to the first surface may be greater than orequal to a thickness of the portion of the emission layer adjacent tothe second surface.

The thickness of the portion of the emission layer adjacent to the first(or the second) surface may be 5 nm, 10 nm, 15 nm, 20 nm, or 25 nm asmeasured from the first (or second) surface.

The charge auxiliary layer may include a hole auxiliary layer 12 betweenthe first electrode 11 and the emission layer 13 or an electronauxiliary layer 14 between the second electrode 15 and the emissionlayer 13.

The light emitting device may further include a substrate. The substratemay be disposed at the side of the first electrode 11 or the secondelectrode 15. In an embodiment, the substrate may be disposed at theside of the first electrode. The substrate may be a substrate includingan insulation material (e.g., insulating transparent substrate). Thesubstrate may include glass; various polymers such as polyester (e.g.,polyethyleneterephthalate (PET), polyethylenenaphthalate (PEN)),polycarbonate, polyacrylate, polyimide, and polyamideimide; polysiloxane(e.g., PDMS); inorganic materials such as Al₂O₃ and ZnO; or acombination thereof, but is not limited thereto. The substrate may bemade of a silicon wafer, and the like. Herein “transparent” may refer tothe case where transmittance of the substrate for light in apredetermined wavelength (e.g., light emitted from the quantum dots) maybe greater than or equal to about 85%, for example, greater than orequal to about 88%, greater than or equal to about 90%, greater than orequal to about 95%, greater than or equal to about 97%, or greater thanor equal to about 99%. A thickness of the substrate may be appropriatelyselected considering a substrate material, and the like, but is notparticularly limited. The transparent substrate may have flexibility.The substrate may be omitted.

One of the first electrode 11 and the second electrode 15 may be ananode and the other may be a cathode. For example, the first electrode11 may be an anode and the second electrode 15 may be a cathode.

The first electrode 11 may be made of a conductor, for example a metal,a conductive metal oxide, or a combination thereof. The first electrode11 may be for example made of a metal, such as nickel, platinum,vanadium, chromium, copper, zinc, and gold, or an alloy thereof; aconductive metal oxide, such as zinc oxide, indium oxide, tin oxide,indium tin oxide (ITO), indium zinc oxide (IZO), or fluorine doped tinoxide; or a combination of metal and oxide, such as ZnO and Al or SnO₂and Sb; and the like, but is not limited thereto. In an embodiment, thefirst electrode may include a transparent conductive metal oxide, forexample, indium tin oxide. A work function of the first electrode may behigher than a work function of the second electrode that will bedescribed later. A work function of the first electrode may be lowerthan a work function of the second electrode that will be describedlater.

The second electrode 15 may be made of a conductor, for example a metal,a conductive metal oxide, a conductive polymer, or a combinationthereof. The second electrode 15 may be for example a metal, such asaluminum, magnesium, calcium, sodium, potassium, titanium, indium,yttrium, lithium, gadolinium, silver, gold, platinum, tin, lead, cesium,or barium, or an alloy thereof; a multi-layer structured material, suchas LiF/Al, Li₂O/Al, Liq/Al, LiF/Ca, and BaF₂/Ca, but is not limitedthereto. The conductive metal oxide is the same as described above.

The work function of the first electrode may be greater than a workfunction of the second electrode. The work function of the firstelectrode may be lower than a work function of the second electrode.

In an embodiment, a work function of the first electrode 11 may be forexample about 4.5 electronvolts (eV) to about 5.0 eV (e.g., about 4.6 eVto about 4.9 eV) and a work function of the second electrode 15 may befor example greater than or equal to about 4.0 eV and less than 4.5 eV(e.g., about 4.0 eV to about 4.3 eV). In an embodiment, the workfunction of the second electrode 15 may be for example about 4.5 eV toabout 5.0 eV (e.g., about 4.6 eV to about 4.9 eV) and the work functionof the first electrode 11 may be for example greater than or equal toabout 4.0 eV and less than about 4.5 eV (e.g., about 4.0 eV to about 4.3eV).

The first electrode 11, the second electrode 15, or a combinationthereof may be a light-transmitting electrode, and thelight-transmitting electrode may be for example made of a conductiveoxide such as a zinc oxide, indium oxide, tin oxide, indium tin oxide(ITO), indium zinc oxide (IZO), or fluorine doped tin oxide, or a metalthin layer of a single layer or a multilayer. When one of the firstelectrode 11 and the second electrode 15 is a non-light-transmittingelectrode, the non-light-transmitting electrode may be made of forexample an opaque conductor such as aluminum (Al), silver (Ag), or gold(Au).

A thickness of the electrodes (the first electrode, the secondelectrode, or a combination thereof) is not particularly limited and maybe appropriately selected considering device efficiency. For example,the thickness of the electrodes may be greater than or equal to about 5nm, for example, greater than or equal to about 50 nm. For example, thethickness of the electrodes may be less than or equal to about 100 μm,for example, less than or equal to about 10 μm, less than or equal toabout 1 μm, less than or equal to about 900 nm, less than or equal toabout 500 nm, or less than or equal to about 100 nm.

The emission layer 13 includes a first surface facing the chargeauxiliary layer and the opposite second surface. The emission layer 13may have a multi-layered structure. The emission layer includes (e.g., aplurality of) quantum dot(s). The quantum dots are nano-sizedsemiconductor nanocrystal particles and exhibit quantum confinementeffects. The quantum dots may include a Group II-VI compound, a GroupIII-V compound, a Group IV-VI compound, a Group IV element or compound,a Group compound, a Group compound, a Group I-II-IV-VI compound, or acombination thereof.

The Group II-VI compound may be a binary element compound such as CdSe,CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, MgSe, MgS, or a combinationthereof; a ternary element compound such as CdSeS, CdSeTe, CdSTe, ZnSeS,ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS,CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, MgZnSe, MgZnS, or a combinationthereof; a quaternary element compound such as ZnSeSTe, HgZnTeS,CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS,HgZnSeTe, HgZnSTe, or a combination thereof; or a combination thereof.The Group II-VI compound may further include a Group III metal. TheGroup III-V compound may be a binary element compound such as GaN, GaP,GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, InSb, or a combinationthereof; a ternary element compound such as GaNP, GaNAs, GaNSb, GaPAs,GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InNP, InNAs, InNSb, InPAs,InPSb, InZnP, or a combination thereof; a quaternary element compoundsuch as GaAlNP, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs,GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, InAlPSb,or a combination thereof; or a combination thereof. The Group III-Vcompound may further include a Group II metal (e.g., InZnP). The GroupIV-VI compound may be a binary element compound such as SnS, SnSe, SnTe,PbS, PbSe, PbTe, or a combination thereof; a ternary element compoundsuch as SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe,SnPbTe, or a combination thereof; a quaternary element compound such asSnPbSSe, SnPbSeTe, SnPbSTe, or a combination thereof; or a combinationthereof. Examples of the Group compound may include CuInSe₂, CuInS₂,CuInGaSe, and CuInGaS, but are not limited thereto. Examples of theGroup I-II-IV-VI compound may include CuZnSnSe and CuZnSnS, but are notlimited thereto. The Group IV element or compound may be an elementarysubstance such as Si, Ge, or a combination thereof; a binary elementcompound such as SiC, SiGe, or a combination thereof; or a combinationthereof.

In an embodiment, the quantum dots may not include a heavy metal (e.g.,cadmium, lead, mercury, or a combination thereof). In an embodiment, thequantum dots may not include cadmium, lead, or a combination thereof. Inan embodiment, the expression “not including the heavy metal” as usedherein may refer to including the heavy metal substantially, forexample, in an amount of less than about 100 parts per million (ppm),less than about 50 ppm, less than about 30 ppm, or less than about 20ppm. The quantum dots may include a Group III-V compound-basedsemiconductor nanocrystal including indium and phosphorus. The GroupIII-V compound may further include zinc. The quantum dots may include asemiconductor nanocrystal including a Group II-VI compound including achalcogen element (e.g., sulfur, selenium, tellurium, or a combinationthereof) and zinc.

In the quantum dots, the aforementioned binary element compound, ternaryelement compound, the quaternary element compound, or a combinationthereof respectively exist in a uniform concentration in thesemiconductor nanocrystal particles or partially differentconcentrations in the same particles. The semiconductor nanocrystals mayhave a core/shell structure wherein a first semiconductor nanocrystal(core) surrounds another second semiconductor nanocrystal (shell) havingthe same or different composition. In an embodiment, the quantum dotsmay include a core including the aforementioned compounds (i.e., GroupII-VI compound, Group III-V compound, Group IV-VI compound, Group IVelement or compound, Group I-III-VI compound, Group compound, GroupI-II-IV-VI compound, or a combination thereof), and a shell having adifferent composition from the core and including the aforementionedcompounds. The core may include InP, InZnP, ZnSe, ZnSeTe, or acombination thereof. The shell may include InP, InZnP, ZnSe, ZnS,ZnSeTe, ZnSeS, or a combination thereof. The shell may include amulti-layered shell having at least two layers. The shell may includeZn, Se, and optionally S (e.g., directly) on the core. The shell mayinclude zinc and sulfur in the outermost layer.

The core and the shell may have an interface, and an element in theinterface may have a concentration gradient wherein the concentration ofthe element of the shell decreases toward the core. The semiconductornanocrystal may have a structure including one semiconductor nanocrystalcore and a multi-layered shell surrounding the same. Herein, themulti-layered shell has at least two shells wherein each shell may havea single composition, an alloy, or the one having a concentrationgradient.

In the quantum dots, the shell material and the core material may havedifferent energy bandgaps from each other. For example, the energybandgaps of the shell material may be greater than that of the corematerial. According to an embodiment, the energy bandgaps of the shellmaterial may be less than that of the core material. The quantum dotsmay have a multi-layered shell. In the multi-layered shell, the energybandgap of the outer layer may be greater than the energy bandgap of theinner layer (i.e., the layer nearer to the core). In the multi-layeredshell, the energy bandgap of the outer layer may be less than the energybandgap of the inner layer.

In an embodiment, the quantum dots may include a core including a firstsemiconductor nanocrystal including indium, phosphorus, and optionallyzinc and a shell disposed on the core and including a secondsemiconductor nanocrystal including zinc and a chalcogen element. In anembodiment, the quantum dots may include a core including a firstsemiconductor nanocrystal including zinc, selenium, and optionallytellurium and a shell disposed on the core and including a secondsemiconductor nanocrystal including zinc and a chalcogen element.

The quantum dots may have a particle size of greater than or equal toabout 1 nm and less than or equal to about 100 nm. The quantum dots mayhave a particle size of about 1 nm to about 20 nm, for example, greaterthan or equal to about 2 nm, greater than or equal to about 3 nm,greater than or equal to about 4 nm, greater than or equal to about 5nm, greater than or equal to about 6 nm, greater than or equal to about7 nm, or greater than or equal to about 8 nm and less than or equal toabout 50 nm, less than or equal to about 40 nm, less than or equal toabout 30 nm, less than or equal to about 20 nm, less than or equal toabout 15 nm, less than or equal to about 10 nm, less than or equal toabout 9 nm, or less than or equal to about 8 nm. The shape of thequantum dots is not particularly limited. For example, the shape of thequantum dots may be a sphere, a polyhedron, a pyramid, a multipod, asquare (cube or cuboid), a rectangular parallelepiped, a nanotube, ananorod, a nanowire, a nanosheet, or a combination thereof, but is notlimited thereto.

The aforementioned quantum dots may be commercially available orappropriately synthesized.

In the light emitting device according to an embodiment, the quantumdots may include a first organic ligand on the surfaces of the quantumdots.

The first organic ligand may have a hydrophobic moiety. The firstorganic ligand may be bound to the surfaces of the quantum dots. Thefirst organic ligand may include RCOOH, RNH₂, R₂NH, R₃N, RSH, R₃PO, R₃P,ROH, RCOOR, RPO(OH)₂, RHPOOH, RHPOOH, or a combination thereof, whereineach R is independently a C3 to C40, or C5 to C40 substituted orunsubstituted aliphatic hydrocarbon group such as a substituted orunsubstituted C3 to C40 alkyl or alkenyl, a C6 to C40 substituted orunsubstituted aromatic hydrocarbon group such as a substituted orunsubstituted C6 to C40 aryl group, or a combination thereof.

Examples of the organic ligand may be a thiol compound such as methanethiol, ethane thiol, propane thiol, butane thiol, pentane thiol, hexanethiol, octane thiol, dodecane thiol, hexadecane thiol, octadecane thiol,or benzyl thiol; an amine compound such as methane amine, ethane amine,propane amine, butane amine, pentyl amine, hexyl amine, octyl amine,nonyl amine, decyl amine, dodecyl amine, hexadecyl amine, octadecylamine, dimethyl amine, diethyl amine, dipropyl amine, tributyl amine, ortrioctyl amine; a carboxylic acid compound such as methanoic acid,ethanoic acid, propanoic acid, butanoic acid, pentanoic acid, hexanoicacid, heptanoic acid, octanoic acid, dodecanoic acid, hexadecanoic acid,octadecanoic acid, oleic acid, or benzoic acid; a phosphine compoundsuch as methyl phosphine, ethyl phosphine, propyl phosphine, butylphosphine, pentyl phosphine, octyl phosphine, dioctyl phosphine,tributyl phosphine, diphenyl phosphine, triphenyl phosphine, or trioctylphosphine; a phosphine oxide compound such as methyl phosphine oxide,ethyl phosphine oxide, propyl phosphine oxide, butyl phosphine oxidepentyl phosphineoxide, tributyl phosphine oxide, octyl phosphine oxide,dioctyl phosphine oxide, diphenyl phosphine oxide, triphenyl phosphineoxide, or trioctyl phosphine oxide; a C5 to C20 alkyl phosphinic acidsuch as hexyl phosphinic acid, octyl phosphinic acid, dodecanephosphinic acid, tetradecane phosphinic acid, hexadecane phosphinicacid, octadecane phosphinic acid; an alkylphosphonic acid such as a C5to C20 alkylphosphonic acid, for example, hexyl phosphonic acid, octylphosphonic acid, dodecane phosphonic acid, tetradecane phosphonic acid,hexadecane phosphonic acid, or octadecane phosphonic acid; and the like,but are not limited thereto. The quantum dots may include a hydrophobicorganic ligand that is the same, or a mixture of at least two differenthydrophobic organic ligands. The hydrophobic organic ligand may notinclude a photopolymerizable moiety (e.g., an acrylate group, amethacrylate group, etc.).

The quantum dots may include an organic ligand and a halogen (e.g.,chlorine) on a surface thereof (hereinafter, also referred to as halogen(Cl)-treated quantum dot).

In the halogen-treated quantum dots, an amount of organics may begreater than or equal to about 1 wt %, for example, greater than orequal to about 2 wt %, greater than or equal to about 3 wt %, greaterthan or equal to about 4 wt %, greater than or equal to about 5 wt %, orgreater than or equal to about 6 wt % and/or less than or equal to about10 wt %, less than or equal to about 9.5 wt %, less than or equal toabout 9 wt %, or less than or equal to about 8 wt % e.g., e.g., asconfirmed by a thermogravimetric analysis.

In the halogen-treated quantum dots, an amount of the halogen may begreater than or equal to about 1 microgram (μg), greater than or equalto about 1.5 μg, greater than or equal to about 3 μg, greater than orequal to about 4 μg, greater than or equal to about 5 μg, greater thanor equal to about 6 μg, greater than or equal to about 7 μg, greaterthan or equal to about 8 μg, greater than or equal to about 9 μg,greater than or equal to about 10 μg, greater than or equal to about 11μg, greater than or equal to about 12 μg, greater than or equal to about12.5 μg, greater than or equal to about 13 μg, greater than or equal toabout 14 μg, greater than or equal to about 15 μg, greater than or equalto about 16 μg, greater than or equal to about 17 μg, greater than orequal to about 18 μg, or greater than or equal to about 19 μg and/orless than about 30 μg, less than or equal to about 25 μg, less than orequal to about 20 μg, less than or equal to about 19.5 μg, less than orequal to about 19 μg, less than or equal to about 18 μg, less than orequal to about 17 μg, less than or equal to about 15 μg, less than orequal to about 12.5 μg, or less than or equal to about 12 μg, per 1milligram (mg) of the quantum dots, e.g., as confirmed by an ionchromatography. The halogen may be chlorine.

The halogen treated quantum dots may be included in the portion adjacentto the first surface (e.g., a first emission layer). The halogen treatedquantum dots may be included in the portion adjacent to the secondsurface (e.g., a second emission layer). In an embodiment, the portionadjacent to the first surface (e.g., a first emission layer) may includethe halogen treated quantum dots and the portion adjacent to the secondsurface (e.g., a second emission layer) may be formed by a treatment ofremoving an organic ligand (e.g., spin dry treatment) that will be setforth below.

The halogen treated quantum dots may be prepared by a method thatincludes:

obtaining an organic dispersion including a plurality of quantum dotsincluding first organic ligands on the surfaces and a first organicsolvent;

obtaining a halide (e.g., chloride) solution including a polar organicsolvent compatible with the first organic solvent and a metal halide;and

adding the halide solution to the organic dispersion so that a contentof the metal halide based on a total weight of the quantum dots may begreater than or equal to about 0.1 wt % and less than or equal to about10 wt %, and stirring the resultant at a temperature of greater than orequal to about 45° C., for example, greater than or equal to about 50°C., greater than or equal to about 55° C., or greater than or equal toabout 60° C. and less than or equal to about 150° C., less than or equalto about 140° C., less than or equal to about 100° C., less than orequal to about 90° C., less than or equal to about 80° C., or less thanor equal to about 70° C., wherein a volume ratio of the polar organicsolvent relative to the first organic solvent is less than or equal toabout 0.1. A volume ratio of the polar organic solvent relative to thefirst organic solvent may be less than or equal to about 0.05. A volumeratio of the polar organic solvent relative to the first organic solventmay be greater than or equal to about 0.001, greater than or equal toabout 0.005, or greater than or equal to about 0.01.

The metal halide (chloride) includes zinc, indium, gallium, magnesium,lithium, or a combination thereof. The first organic solvent may includea substituted or unsubstituted C5 to C40 aliphatic hydrocarbon, asubstituted or unsubstituted C6 to C40 aromatic hydrocarbon, a C3 to C40alicyclic hydrocarbon, or a combination thereof. The polar organicsolvent may include a C1 to C10 alcohol, or a combination thereof.

The emission layer 13 may not include a thiol organic ligand (e.g.,represented by RSH wherein, R is independently a C3 to C40 substitutedor unsubstituted aliphatic hydrocarbon group, a C6 to C40 substituted orunsubstituted aromatic hydrocarbon group, or a combination thereof).

The emission layer 13 may include a first surface facing the chargeauxiliary layer (e.g., electron auxiliary layer or hole auxiliary layer)that will be described later and an opposite second surface. In theemission layer, the amount (e.g., the concentration) of the organicligand in the portion adjacent to the first surface may be larger thanthe amount of the organic ligand at the portion adjacent to the secondsurface. The charge auxiliary layer may be the electron auxiliary layer14. In an embodiment, a hole transport capability (or hole mobility) inthe portion adjacent to the second surface may be greater than a holetransport ability (or hole mobility) in the portion adjacent to thefirst surface.

Herein, a thickness of the portion adjacent to the first surface or thesecond surface may be greater than or equal to about 1 nm, for example,greater than or equal to about 2 nm, greater than or equal to about 3nm, greater than or equal to about 4 nm, greater than or equal to about5 nm, greater than or equal to about 6 nm, greater than or equal toabout 7 nm, greater than or equal to about 8 nm, greater than or equalto about 9 nm, greater than or equal to about 10 nm, greater than orequal to about 11 nm, greater than or equal to about 12 nm, greater thanor equal to about 13 nm, greater than or equal to about 14 nm, greaterthan or equal to about 15 nm, greater than or equal to about 16 nm,greater than or equal to about 17 nm, greater than or equal to about 18nm, greater than or equal to about 19 nm, or greater than or equal toabout 20 nm. A thickness of the portion adjacent to the first surface orthe second surface may be less than or equal to about 100 nm, less thanor equal to about 90 nm, less than or equal to about 80 nm, less than orequal to about 70 nm, less than or equal to about 60 nm, less than orequal to about 50 nm, less than or equal to about 40 nm, less than orequal to about 30 nm, or less than or equal to about 20 nm. The portionadjacent to the first surface or the portion adjacent to the secondsurface may correspond to each of the first emission layer 13 b or thesecond emission layer 13 a that will be described later.

The quantum dots may provide high color reproducibility and may be usedas a next-generation display material in terms of forming an emissionlayer in a solution process. Colloid synthesized quantum dots mayinclude organic ligands (e.g., organic compounds including long-chainaliphatic hydrocarbon and a functional group, such as oleic acid (OA))on the surfaces. Such an organic ligand may help dispersibility of thequantum dots in mediums, but the organic ligand may interfere withcharge flows in the quantum dots formed as a monolayer. Accordingly, itmay be difficult to balance the electrons/holes in theelectroluminescent device including the quantum dot emission layer. Forexample, when a flow of positive charges (holes) relative to negativecharges (electrons) is limited in the quantum dot emission layer, alight emitting region may be produced not inside the emission layer buton an interface between a hole auxiliary layer (e.g., hole transportlayer (HTL)) and a QD layer, and excitons produced on the interface maybe extinguished relatively easily, which may have a negative influenceon device efficiency. According to the present inventors' research,extra electrons not recombined on the interface due to high lowestunoccupied molecular orbital (LUMO) energy of QD in QD-LED emitting bluelight may move toward the hole transport layer (HTL), and accordingly,the device efficiency may be more severely deteriorated.

However, a light emitting device according to an embodiment has anemission layer having the aforementioned structure and thus may showprolonged life-span characteristics as well as improvedelectroluminescence properties. Without being bound by any particulartheory, the aforementioned structure of the light emitting deviceaccording to an embodiment may induce the light emitting region producedthrough an electron-hole recombination to be formed in the center of theemission layer (EML), and accordingly, the device may show improvedproperties.

The present inventors have found that an amount change of the organicligands (or the halogen or the chlorine that will be described below)may have a direct influence on hole (or charge) transport capability ofthe emission layer, and accordingly, when an amount of the organicligands in the emission layer is adjusted as described above, theelectron-hole recombination may be formed in the center of the emissionlayer.

In an embodiment, an emission layer including a relatively small amountof the organic ligands (e.g., oleic acid) (or a relatively large amountof the halogen or the chlorine) may show improved hole transfer (HT)characteristics (e.g., hole mobility). Accordingly, when quantum dotshaving a relatively small (less) amount of the organic ligands (or agreater amount of the halogen or the chlorine) are formed to face thehole auxiliary layer, and quantum dots having a relatively large amountof the organic ligands (e.g., oleic acid) (or a less amount of thehalogen or the chlorine) are formed to face a charge (electron)auxiliary layer, the device may show improved electroluminescenceproperties. In other words, changing an amount of the organic ligands(the halogen or the chlorine) in a thickness direction may make itpossible to dispose the quantum dots having improved hole transportcharacteristics near to the hole transport layer (HTL) and to disposethe quantum dots having improved electron transport characteristicsdisposed near to the electron transport layer (ETL). Accordingly, thequantum dot emission layer may have a changing amount of organic ligandsin a thickness direction through a manufacturing method described laterand whereby the light emitting region may move toward the center of thelayer and the device according to an embodiment may show improvedelectroluminescence properties (efficiency and luminance) and aprolonged life-span.

In an embodiment, the amount of the organic ligand at the portionadjacent to the first surface may be larger than the amount of theorganic ligand at the portion adjacent to the second surface by at leastabout 20%. In an embodiment, (e.g., based on a total weight of thequantum dots) colloid quantum dots prepared in an organic solvent underthe presence of, e.g., with, the organic ligand may include an organicmaterial in an amount of at least about 10%, for example, greater thanor equal to about 15%, greater than or equal to about 20%, and less thanor equal to about 50%, less than or equal to about 30%, or less than orequal to about 20% e.g., bound to the surfaces thereof. A surfacetreatment using a halogen compound may change (reduce) the amount of theorganic ligand of the quantum dots.

Thus, in an embodiment, as determined by an analysis such asthermogravimetric analysis, an amount, e.g., a total weight of anorganic material (e.g., the amount or weight of the organic ligand atthe portion adjacent to the first surface, hereinafter referred to as a“first amount”) of the colloid quantum dots may be greater than or equalto about 5%, greater than or equal to about 8%, greater than or equal toabout 10%, greater than or equal to about 15%, greater than or equal toabout 16%, greater than or equal to about 17%, greater than or equal toabout 18%, greater than or equal to about 19%, or greater than or equalto about 20% based on a total weight of the quantum dots. The firstamount may be less than or equal to about 50%, less than or equal toabout 40%, less than or equal to about 30%, less than or equal to about20%, less than or equal to about 15%, or less than or equal to about 10%based on a total weight of the quantum dots.

The first amount may be greater than an amount of the organic ligandpresent in the portion adjacent to the second surface (e.g., the secondamount) by at least about 5%, at least about 10%, at least about 20%, atleast about 30%, at least about 40%, at least about 50%, at least about60%, or at least about 70% in comparison with the second amount.

The second amount may be greater than or equal to about 0.5 wt %,greater than or equal to about 1 wt %, greater than or equal to about 2wt %, greater than or equal to about 3 wt %, greater than or equal toabout 4 wt %, greater than or equal to about 5 wt %, greater than orequal to about 6 wt %, greater than or equal to about 7 wt %, greaterthan or equal to about 8 wt %, greater than or equal to about 9 wt %,greater than or equal to about 10 wt %, greater than or equal to about10.5 wt %, greater than or equal to about 11 wt %, greater than or equalto about 11.5 wt %, greater than or equal to about 12 wt %, greater thanor equal to about 12.5 wt % and less than or equal to about 20 wt %,less than or equal to about 19 wt %, less than or equal to about 18 wt%, less than or equal to about 17 wt %, less than or equal to about 16.7wt %, 16 wt %, less than or equal to about 15.4 wt %, less than or equalto about 15 wt %, less than or equal to about 14 wt %, less than orequal to about 13 wt %, less than or equal to about 12 wt %, less thanor equal to about 10.5% wt %, less than or equal to about 10 wt %, lessthan or equal to about 9 wt %, less than or equal to about 8 wt %, lessthan or equal to about 7 wt %, less than or equal to about 6 wt %, orless than or equal to about 5 wt %, based on a total weight of thequantum dots.

In an embodiment, the first amount may be greater than or equal to about1.01 times, greater than or equal to about 1.02 times, greater than orequal to about 1.03 times, greater than or equal to about 1.04 times,greater than or equal to about 1.05 times, greater than or equal toabout 1.06 times, greater than or equal to about 1.07 times, greaterthan or equal to about 1.08 times, greater than or equal to about 1.09times, greater than or equal to about 1.1 times, greater than or equalto about 1.2 times, greater than or equal to about 1.3 times, greaterthan or equal to about 1.4 times, greater than or equal to about 1.5times, greater than or equal to about 1.6 times, greater than or equalto about 1.7 times, greater than or equal to about 1.8 times, or greaterthan or equal to about 1.9 times as large as the second amount. Thefirst amount may be less than or equal to about 3 times, less than orequal to about 2.9 times, less than or equal to about 2.8 times, lessthan or equal to about 2.7 times, less than or equal to about 2.6 times,less than or equal to about 2.5 times, less than or equal to about 2.4times, less than or equal to about 2.3 times, less than or equal toabout 2.2 times, or less than or equal to about 2.1 times as large assecond amount. Herein, a difference between the amounts of the organicligands may be for example confirmed by scanning or transmissionelectron microscope energy dispersive X-ray spectroscopy (e.g., scanningelectron microscope energy dispersive spectroscopy (SEM-EDX)), and thelike, but is not limited thereto. The difference of the organic ligandmay be confirmed by the comparison of the carbon content of the quantumdots.

In an embodiment, when confirmed by infrared spectroscopy, a peakintensity of a functional group (e.g., carboxylic acid group) of theorganic ligand at the portion adjacent to the first surface may behigher than a peak intensity of a functional group (e.g., carboxylicacid group) of the organic ligand at the portion adjacent to the secondsurface by at least about 20%, for example, at least about 21%, at leastabout 22%, at least about 23%, at least about 24%, at least about 25%,at least about 26%, at least about 27%, at least about 28%, at leastabout 29%, or at least about 30%. The peak intensity may represent anamount (e.g., a weight or a number of moles) of the organic ligand ateach portion.

In an embodiment, when confirmed by X-ray photoelectron spectroscopy(XPS), in the (first or second) emission layer, a ratio of a peak area(or an amount) of carbon relative to a metal (e.g., zinc) of the quantumdot outermost shell may be less than or equal to about 10:1, less thanor equal to about 9, less than or equal to about 8:1, less than or equalto about 7:1, less than or equal to about 6, less than or equal to about5:1, less than or equal to about 4:1, less than or equal to about 3:1,less than or equal to about 2.9:1, less than or equal to about 2.8:1,less than or equal to about 2.7:1, less than or equal to about 2.67:1,less than or equal to about 2.6:1, less than or equal to about 2.5:1,less than or equal to about 2.4:1, less than or equal to about 2.3:1,less than or equal to about 2.2:1, less than or equal to about 2.1:1,less than or equal to about 2:1, less than or equal to about 1.9:1, lessthan or equal to about 1.8:1, less than or equal to about 1.7:1, lessthan or equal to about 1.6:1, less than or equal to about 1.5:1, lessthan or equal to about 1.4:1, or less than or equal to about 1.3:1. Inthe (first or second) emission layer, a ratio of a peak area (or anamount) carbon relative to a metal (e.g., zinc) of the quantum dotoutermost shell may be greater than or equal to about 0.1, greater thanor equal to about 0.2, greater than or equal to about 0.3, greater thanor equal to about 0.4, greater than or equal to about 0.5, greater thanor equal to about 0.6, greater than or equal to about 0.7, greater thanor equal to about 0.8, greater than or equal to about 0.9, greater thanor equal to about 1, greater than or equal to about 1.1, greater than orequal to about 1.2, greater than or equal to about 1.3, greater than orequal to about 1.4, or greater than or equal to about 1.5.

For example, when confirmed by XPS, the emission layer according to anembodiment may exhibit a peak of a compound (e.g., zinc chloride)including a metal (e.g., zinc) and a halogen in the outermost shell ofthe quantum dots.

For example, when confirmed by SEM-EDX, in the emission layer, an amountof a non-metal (e.g., sulfur) relative to a metal (e.g., zinc) of thequantum dot outermost shell (e.g., as for the quantum dot in the portionadjacent to the second surface) may be detected. In the emission layer,an amount of non-metal (e.g., sulfur) relative to a metal (e.g., zinc)of the quantum dot outermost shell may be detected.

In the emission layer, the portion adjacent to the second surface(and/or the portion adjacent to the first surface) may further include ahalogen. The halogen may include fluorine, chlorine, bromine, iodine, ora combination thereof. The presence of the halogen may be confirmed byX-ray photoelectron spectroscopy (XPS), but is not limited thereto. Thepresence of a bond of ZnCl₂ may be confirmed by the XPS of the emissionlayer. An amount of the halogen (e.g., confirmed by XPS or SEM-EDX) ofthe (first or second) emission layer may be greater than or equal toabout 0.0001, for example, greater than or equal to about 0.0005,greater than or equal to about 0.001, greater than or equal to about0.002, greater than or equal to about 0.003, greater than or equal toabout 0.004, greater than or equal to about 0.005, greater than or equalto about 0.006, greater than or equal to about 0.007, greater than orequal to about 0.008, greater than or equal to about 0.009, greater thanor equal to about 0.01, greater than or equal to about 0.02, greaterthan or equal to about 0.03, greater than or equal to about 0.04,greater than or equal to about 0.05, greater than or equal to about0.06, greater than or equal to about 0.07, greater than or equal toabout 0.08, greater than or equal to about 0.09, or greater than orequal to about 0.1, relative to Zn.

An amount of the halogen (e.g., confirmed by XPS or SEM-EDX) of the(first or second) emission layer may be less than or equal to about 0.9,for example, less than or equal to about 0.8, less than or equal toabout 0.7, less than or equal to about 0.6, less than or equal to about0.5, less than or equal to about 0.4, less than or equal to about 0.3,less than or equal to about 0.2, less than or equal to about 0.1, lessthan or equal to about 0.09, less than or equal to about 0.08, less thanor equal to about 0.07, less than or equal to about 0.06, less than orequal to about 0.05, less than or equal to about 0.04, or less than orequal to about 0.03, relative to Zn.

In the emission layer, the quantum dots may control anabsorption/emission wavelength by adjusting a composition and a sizethereof. A maximum peak emission wavelength of the quantum dot may be anultraviolet (UV) to infrared wavelength or a wavelength of greater thanthe above wavelength range. For example, the maximum peak emissionwavelength of the quantum dot may be greater than or equal to about, 300nm, for example, greater than or equal to about 500 nm, greater than orequal to about 510 nm, greater than or equal to about 520 nm, greaterthan or equal to about 530 nm, greater than or equal to about 540 nm,greater than or equal to about 550 nm, greater than or equal to about560 nm, greater than or equal to about 570 nm, greater than or equal toabout 580 nm, greater than or equal to about 590 nm, greater than orequal to about 600 nm, or greater than or equal to about 610 nm. Themaximum peak emission wavelength of the quantum dot may be less than orequal to about 800 nm, for example, less than or equal to about 650 nm,less than or equal to about 640 nm, less than or equal to about 630 nm,less than or equal to about 620 nm, less than or equal to about 610 nm,less than or equal to about 600 nm, less than or equal to about 590 nm,less than or equal to about 580 nm, less than or equal to about 570 nm,less than or equal to about 560 nm, less than or equal to about 550 nm,or less than or equal to about 540 nm. The maximum peak emissionwavelength of the quantum dots may be in the range of about 500 nm toabout 650 nm. The maximum peak emission wavelength of the quantum dotsmay be in the range of about 500 nm to about 550 nm (green). The maximumpeak emission wavelength of the quantum dots may be in the range ofabout 600 nm to about 650 nm (red). In an embodiment, the quantum dotsin the emission layer may be configured to emit light having the samecolor. For example, in the emission layer 13, the quantum dots at theportion adjacent to the first surface may be configured to emit lighthaving the same color as the light emitted by the quantum dots at theportion adjacent to the second surface. Herein, a difference between thecenter wavelengths of these quantum dots may be about 15 nm at maximum,for example, less than or equal to about 10 nm, and in this case, a fullwidth at half maximum (FWHM) of light (e.g., electroluminescence peak)emitted from the emission layer may be less than or equal to about 60nm, less than or equal to about 50 nm, less than or equal to about 40nm, less than or equal to about 35 nm, less than or equal to about 30nm, less than or equal to about 25 nm, or less than or equal to about 20nm.

In the emission layer 13, the quantum dots at the portion adjacent tothe first surface may be configured to emit light having a differentcolor from the quantum dots at the portion adjacent to the secondsurface. For example, the quantum dots at the portion adjacent to thefirst surface may have a maximum peak emission wavelength in a green (orred) range and the quantum dots at the portion adjacent to the secondsurface may have a maximum peak emission wavelength in a red (or green)range.

The quantum dots may have (electroluminescence or photoluminescence)quantum efficiency of greater than or equal to about 10%, for example,greater than or equal to about 30%, greater than or equal to about 50%,greater than or equal to about 60%, greater than or equal to about 70%,greater than or equal to about 90%, or about 100%. The quantum dots mayhave a relatively narrow emission spectrum. A (electro- or photo-)emission spectrum of the quantum dots may have for example a full widthat half maximum (FWHM) of less than or equal to about 50 nm, for exampleless than or equal to about 45 nm, less than or equal to about 40 nm,less than or equal to about 35 nm, or less than or equal to about 30 nm.

The emission layer 13 may have a thickness of greater than or equal toabout 5 nm, for example, greater than or equal to about 10 nm, greaterthan or equal to about 20 nm, or greater than or equal to about 30 nmand less than or equal to about 200 nm, for example, less than or equalto about 150 nm, less than or equal to about 100 nm, less than or equalto about 90 nm, less than or equal to about 80 nm, less than or equal toabout 70 nm, less than or equal to about 60 nm, or less than or equal toabout 50 nm. The emission layer 13 may have for example a thickness ofabout 10 nm to about 150 nm, for example about 10 nm to about 100 nm orabout 10 nm to about 50 nm.

In an embodiment, the emission layer may include a first emission layerincluding the first surface 13 b (e.g., facing the electron auxiliarylayer) and a second emission layer including the second surface 13 a(e.g., facing the hole auxiliary layer). In an embodiment, the emissionlayer may include a first emission layer including the first surface(e.g., facing the hole auxiliary layer) and a second emission layerincluding the second surface (e.g., facing the electron auxiliarylayer). Each of the first emission layer and the second emission layermay include the first organic ligand. The first emission layer maycorrespond to the portion adjacent to the first surface. The secondemission layer may correspond to the portion adjacent to the secondsurface.

A thickness of the first emission layer may be greater than or equal toabout 3 nm, for example, greater than or equal to about 5 nm, greaterthan or equal to about 10 nm, greater than or equal to about 15 nm,greater than or equal to about 16 nm, greater than or equal to about 17nm, greater than or equal to about 18 nm, greater than or equal to about19 nm, greater than or equal to about 20 nm, greater than or equal toabout 21 nm, greater than or equal to about 22 nm, greater than or equalto about 23 nm, greater than or equal to about 24 nm, or greater than orequal to about 25 nm. A thickness of the first emission layer may beless than or equal to about 100 nm, for example, less than or equal toabout 90 nm, less than or equal to about 80 nm, less than or equal toabout 70 nm, less than or equal to about 60 nm, less than or equal toabout 50 nm, less than or equal to about 40 nm, less than or equal toabout 30 nm, less than or equal to about 25 nm, or less than or equal toabout 20 nm.

In an embodiment, the thickness of the first emission layer may be atleast 1 monolayer (e.g., at least 2 monolayers) consisting of quantumdots, but is not limited thereto.

A thickness of the second emission layer may be greater than or equal toabout 3 nm, for example, greater than or equal to about 5 nm, greaterthan or equal to about 10 nm, greater than or equal to about 15 nm,greater than or equal to about 16 nm, greater than or equal to about 17nm, greater than or equal to about 18 nm, greater than or equal to about19 nm, greater than or equal to about 20 nm, greater than or equal toabout 21 nm, greater than or equal to about 22 nm, greater than or equalto about 23 nm, greater than or equal to about 24 nm, or greater than orequal to about 25 nm. A thickness of the second emission layer may beless than or equal to about 100 nm, for example, 90 nm, less than orequal to about 80 nm, less than or equal to about 70 nm, less than orequal to about 60 nm, less than or equal to about 50 nm, less than orequal to about 40 nm, less than or equal to about 30 nm, less than orequal to about 25 nm or less than or equal to about 20 nm. In anembodiment, the thickness of the second emission layer may be 1monolayer or more (e.g., 2 monolayers) consisting of quantum dots, butis not limited thereto.

The first emission layer and the second emission layer may include thesame types of organic ligand. In an embodiment, the first emission layerand the second emission layer may include an organic ligand having acarboxylic acid group. In an embodiment, the first emission layer mayinclude an organic ligand having a carboxylic acid moiety, an organicligand having a thiol group, or a combination thereof, and the secondemission layer may be an organic ligand having a carboxylic acid moiety.The second emission layer and/or the first emission layer may notinclude an organic ligand having a thiol group.

The first emission layer may include the organic ligand included in thesecond emission layer and may further include an organic ligand that isnot included in the second emission layer. The second emission layer mayfurther include a halogen. The halogen included in the emission layer isthe same as described above.

The second emission layer may be (substantially) insoluble to a C1 toC10 alcohol solvent (e.g., ethanol, methanol, propanol, isopropanol,butanol, pentanol, isopentanol, hexanol, heptanol, etc.), cyclohexylacetate, acetone, toluene, cyclohexane, a C1 to C10 alkane seriessolvent (e.g., hexane), or a combination thereof.

The first emission layer may be substantially insoluble to a C1 to C10alcohol solvent.

The second emission layer may not include arylamine. The first emissionlayer may not include an organic compound having a heterocycle includingoxygen, sulfur, nitrogen, or silicon.

The second emission layer may further include a second organic ligandthat is different from the first organic ligand, and the second organicligand may include a C3 to C20 organic compound having a thiol group andan alcohol group. In an embodiment, the second emission layer may notinclude a thiol organic ligand as described above.

The first emission layer may further include a second organic ligandthat is different from the first organic ligand, and the second organicligand may further include a C3 to C40 alkanethiol. In an embodiment,the first emission layer may not include a thiol organic ligand.

The emission layer may further include a third emission layer includingquantum dots having different electrical properties from the first andsecond emission layers between the first emission layer 13 b and thesecond emission layer 13 a. The quantum dots included in the thirdemission layer may further include metal oxide (e.g., aluminum oxide,etc.) on the surface.

The emission layer 13 (e.g., the first emission layer, the secondemission layer, or the combination thereof) may have a HOMO energy levelof greater than or equal to about 5.4 eV, greater than or equal to about5.6 eV, greater than or equal to about 5.7 eV, greater than or equal toabout 5.8 eV, greater than or equal to about 5.9 eV, or greater than orequal to about 6.0 eV. The emission layer 13 may have a HOMO energylevel of less than or equal to about 7.0 eV, less than or equal to about6.8 eV, less than or equal to about 6.7 eV, less than or equal to about6.5 eV, less than or equal to about 6.3 eV, or less than or equal toabout 6.2 eV. In an embodiment, the emission layer 13 may have a HOMOenergy level of about 5.5 eV to about 6.1 eV.

The emission layer 13 (e.g., the first emission layer, the secondemission layer, or the combination thereof) may have for example an LUMOenergy level of less than or equal to about 3.8 eV, less than or equalto about 3.7 eV, less than or equal to about 3.6 eV, less than or equalto about 3.5 eV, less than or equal to about 3.4 eV, less than or equalto about 3.3 eV, less than or equal to about 3.2 eV, or less than orequal to about 3.0 eV. The emission layer 13 may have an LUMO energylevel of greater than or equal to about 2.5 eV, for example, greaterthan or equal to about 2.6 eV, greater than or equal to about 2.7 eV, orgreater than or equal to about 2.8 eV. In an embodiment, the emissionlayer 13 may have an energy bandgap of about 2.4 eV to about 3.5 eV.

The light emitting device according to an embodiment may include acharge auxiliary layer. The charge auxiliary layer may include anelectron auxiliary layer, a hole auxiliary layer, or a combinationthereof.

The hole auxiliary layer 12 may be disposed between the first electrode11 (e.g., anode) and the emission layer 13. The hole auxiliary layer 12may have one layer or two or more layers and may include, for example ahole injection layer, a hole transport layer, an electron blockinglayer, or a combination thereof.

The hole auxiliary layer 12 may have a HOMO energy level that matches aHOMO energy level of the emission layer 13 and mobility of holes fromthe hole auxiliary layer 12 into the emission layer 13 may be aided.

The HOMO energy level of the hole auxiliary layer 12 (e.g., holetransport layer (HTL)) contacting the emission layer may be the same asor smaller than the HOMO energy level of the emission layer 13 by avalue within a range of less than or equal to about 1.0 eV. For example,a difference of HOMO energy levels between the hole auxiliary layer 12and the emission layer 13 may be about 0 eV to about 1.0 eV, for exampleabout 0.01 eV to about 0.8 eV, about 0.01 eV to about 0.7 eV, about 0.01eV to about 0.5 eV, about 0.01 eV to about 0.4 eV, about 0.01 eV toabout 0.3 eV, about 0.01 eV to about 0.2 eV, or about 0.01 eV to about0.1 eV.

The HOMO energy level of the hole auxiliary layer 12 may be for examplegreater than or equal to about 5.0 eV, for example, greater than orequal to about 5.2 eV, greater than or equal to about 5.4 eV, greaterthan or equal to about 5.6 eV, or greater than or equal to about 5.8 eV.For example, the HOMO energy level of the hole auxiliary layer 12 may beabout 5.0 eV to about 7.0 eV, about 5.2 eV to 6.8 eV, about 5.4 eV toabout 6.8 eV, about 5.4 eV to about 6.7 eV, about 5.4 eV to about 6.5eV, about 5.4 eV to about 6.3 eV, about 5.4 eV to about 6.2 eV, about5.4 eV to about 6.1 eV, about 5.6 eV to about 7.0 eV, about 5.6 eV toabout 6.8 eV, about 5.6 eV to about 6.7 eV, about 5.6 eV to about 6.5eV, about 5.6 eV to about 6.3 eV, about 5.6 eV to about 6.2 eV, about5.6 eV to about 6.1 eV, about 5.8 eV to about 7.0 eV, about 5.8 eV toabout 6.8 eV, about 5.8 eV to about 6.7 eV, about 5.8 eV to about 6.5eV, about 5.8 eV to about 6.3 eV, about 5.8 eV to about 6.2 eV, or about5.8 eV to about 6.1 eV.

In an embodiment, the hole auxiliary layer 12 may include a holeinjection layer near to the first electrode 11 and a hole transportlayer near to the emission layer 13. Herein, the HOMO energy level ofthe hole injection layer may be about 5.0 eV to about 6.0 eV, about 5.0eV to about 5.5 eV, about 5.0 eV to about 5.4 eV and the HOMO energylevel of the hole transport layer may be about 5.2 eV to about 7.0 eV,about 5.4 eV to about 6.8 eV, about 5.4 eV to about 6.7 eV, about 5.4 eVto about 6.5 eV, about 5.4 eV to about 6.3 eV, about 5.4 eV to about 6.2eV, or about 5.4 eV to about 6.1 eV.

A material included in the hole auxiliary layer 12 (e.g., hole transportlayer or hole injection layer) is not particularly limited and mayinclude for examplepoly[(9,9-dioctyl-fluorene-2,7-diyl)-co-(N-(4-butylphenyl)-diphenylamine)](TFB),polyarylamine, poly(N-vinylcarbazole, poly(3,4-ethylenedioxythiophene)(PEDOT), poly(3,4-ethylenedioxythiophene)polystyrene sulfonate(PEDOT:PSS), polyaniline, polypyrrole,N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (TPD),4-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD), m-MTDATA(4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine),4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA), 1,1-bis[(di-4-tolylamino)phenylcyclohexane (TAPC),diipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile(HAT-CN), p-type metal oxide (e.g., NiO, WO₃, MoO₃, etc.), acarbon-based material such as graphene oxide, or a combination thereof,but is not limited thereto.

In the hole auxiliary layer(s), a thickness of each layer may beappropriately selected. For example, the thickness of each layer may begreater than or equal to about 10 nm, for example, greater than or equalto about 15 nm, greater than or equal to about 20 nm, and less than orequal to about 100 nm, for example, less than or equal to about 90 nm,less than or equal to about 80 nm, less than or equal to about 70 nm,less than or equal to about 60 nm, less than or equal to about 50 nm,less than or equal to about 40 nm, less than or equal to about 35 nm, orless than or equal to about 30 nm, but is not limited thereto.

The electron auxiliary layer 14 may be disposed between the emissionlayer 13 and the second electrode 15 (e.g., cathode). The electronauxiliary layer 14 may include for example an electron injection layer,an electron transport layer, a hole blocking layer, or a combinationthereof, but is not limited thereto. In an embodiment, the electronauxiliary layer 14 may include an electron transport layer.

The electron transport layer, the electron injection layer, or acombination thereof may include for example1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA), bathocuproine(BCP), tris[3-(3-pyridyl)-mesityl]borane (3TPYMB), LiF, Alq₃, Gaq₃,Inq₃, Znq₂, Zn(BTZ)₂, BeBq₂, ET₂O₄(8-(4-(4,6-di(naphthalen-2-yl)-1,3,5-triazin-2-yl)phenyl)quinolone),8-hydroxyquinolinato lithium (Liq), n-type metal oxide (e.g., ZnO, HfO₂,etc.), or a combination thereof, but is not limited thereto. The holeblocking layer may include for example1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA), bathocuproine(BCP), tris[3-(3-pyridyl)-mesityl]borane (3TPYMB), LiF, Alq₃, Gaq₃,Inq₃, Znq₂, Zn(BTZ)₂, BeBq₂, or a combination thereof, but is notlimited thereto.

In an embodiment, the electron auxiliary layer 14 (e.g., electrontransport layer) includes a plurality of nanoparticles. Thenanoparticles include a metal oxide including zinc.

The metal oxide may include Zn_(1-x)M_(x)O (wherein M is Mg, Ca, Zr, W,Li, Ti, or a combination thereof and 0≤x≤0.5). In an embodiment, inChemical Formula 1, M may be magnesium (Mg). In an embodiment, inChemical Formula 1, x may be greater than or equal to about 0.01 andless than or equal to about 0.3, for example, less than or equal toabout 0.25, less than or equal to about 0.2, or less than or equal toabout 0.15.

The metal oxide may include zinc oxide, zinc magnesium oxide, or acombination thereof. An absolute value of LUMO of quantum dots includedin the emission layer may be smaller than an absolute value of LUMO ofthe metal oxide. In an embodiment, an absolute value of LUMO of quantumdots may be larger than an absolute value of LUMO of a metal oxide ETL.An absolute value of LUMO of blue QD may be smaller than an absolutevalue of LUMO of a metal oxide ETL. Electron injection in anelectroluminescent device including blue QD may be different from alight emitting device including red or green quantum dots.

An average size of the nanoparticles may be greater than or equal toabout 1 nm, for example, greater than or equal to about 1.5 nm, greaterthan or equal to about 2 nm, greater than or equal to about 2.5 nm, orgreater than or equal to about 3 nm and less than or equal to about 10nm, less than or equal to about 9 nm, less than or equal to about 8 nm,less than or equal to about 7 nm, less than or equal to about 6 nm, orless than or equal to about 5 nm. The nanoparticles may not have a rodshape. The nanoparticles may not have a nano wire shape.

In an embodiment, each thickness of the electron auxiliary layer 14(e.g., an electron injection layer, an electron transport layer, or ahole blocking layer) may be greater than or equal to about 5 nm, greaterthan or equal to about 6 nm, greater than or equal to about 7 nm,greater than or equal to about 8 nm, greater than or equal to about 9nm, greater than or equal to about 10 nm, greater than or equal to about11 nm, greater than or equal to about 12 nm, greater than or equal toabout 13 nm, greater than or equal to about 14 nm, greater than or equalto about 15 nm, greater than or equal to about 16 nm, greater than orequal to about 17 nm, greater than or equal to about 18 nm, greater thanor equal to about 19 nm, or greater than or equal to about 20 nm andless than or equal to about 120 nm, less than or equal to about 110 nm,less than or equal to about 100 nm, less than or equal to about 90 nm,less than or equal to about 80 nm, less than or equal to about 70 nm,less than or equal to about 60 nm, less than or equal to about 50 nm,less than or equal to about 40 nm, less than or equal to about 30 nm, orless than or equal to about 25 nm, but is not limited thereto.

A device according to an embodiment may have a normal structure. In anembodiment, in the device, an anode 10 disposed on a transparentsubstrate 100 may include a metal oxide-based transparent electrode(e.g., ITO electrode) and a cathode 50 facing the anode 10 may include aconductive metal (e.g., Mg, Al, Ag, or a combination thereof) (e.g.,having a relatively low work function). A hole auxiliary layer 20 (e.g.,a hole injection layer of PEDOT:PSS, p-type metal oxide, or acombination thereof, and the like; a hole transport layer of TFB,poly(N-vinylcarbazole) (PVK), or a combination thereof; or a combinationthereof) may be disposed between the transparent electrode 10 and thequantum dot emission layer 30. The hole injection layer may be near tothe transparent electrode and the hole transport layer may be near tothe emission layer. An electron auxiliary layer 40 such as an electroninjection layer/an electron transport layer may be disposed between thequantum dot emission layer 30 and the cathode 50. (see FIG. 2)

A device according to an embodiment may have an inverted structure. Acathode 50 disposed on the transparent substrate 100 may include a metaloxide-based transparent electrode (e.g., ITO) and an anode 10 facing thecathode may include a metal (e.g., Au, Ag, Al, Mg, or a combinationthereof) (e.g., having a relatively high work function). For example,(optionally doped) n-type metal oxide (crystalline Zn metal oxide) maybe disposed between the transparent electrode 50 and the emission layer30 as an electron auxiliary layer 40 (e.g., electron transport layer).MoO₃ or other p-type metal oxides may be disposed between the metalanode 10 and the quantum dot emission layer 30 as a hole auxiliary layer20 (e.g., a hole transport layer including TFB, PVK, or a combinationthereof; a hole injection layer including MoO₃ or other p-type metaloxides; or a combination thereof). (refer to FIG. 3)

An embodiment provides a method of manufacturing the aforementionedlight emitting device. The manufacturing method may include forming anemission layer on a first electrode; forming a charge auxiliary layer onthe emission layer; and forming a second electrode on the chargeauxiliary layer, wherein the forming of the emission layer includesforming a first layer including a plurality of quantum dots having afirst organic ligand on the surface; removing at least a portion of(e.g., a portion of) the first organic ligand from the first layer; andforming a second layer (e.g., first emission layer) including aplurality of quantum dots having a first organic ligand on the surface,on the ligand-treated first layer (e.g., second emission layer).

The charge auxiliary layer may be an electron auxiliary layer.

The method may further include forming a charge auxiliary layer (e.g.,hole auxiliary layer) on the first electrode before forming the emissionlayer on the first electrode. In this case, the emission layer may beformed on the charge auxiliary layer disposed on the first electrode.

The first electrode, the emission layer, the charge auxiliary layer, andthe second electrode are the same as described above.

The forming of the emission layer may be performed by dispersing thequantum dots in a solvent (e.g., organic solvent) to obtain a quantumdot dispersion and applying or depositing the quantum dot dispersion onthe substrate or the charge auxiliary layer in an appropriate manner(e.g., spin coating, inkjet printing, etc.). The forming of the emissionlayer may further include heat-treating the applied or deposited quantumdot layer. The heat-treating temperature is not particularly limited,and may be appropriately selected considering a boiling point of theorganic solvent. For example, the heat-treating temperature may begreater than or equal to about 60° C. The organic solvent of the quantumdot dispersion is not particularly limited and thus may be appropriatelyselected. In an embodiment, the organic solvent may include a(substituted or unsubstituted) aliphatic hydrocarbon organic solvent, a(substituted or unsubstituted) aromatic hydrocarbon organic solvent, anacetate solvent, or a combination thereof.

The removing of the ligand from the formed first layer may includepreparing an alcohol solution of a metal halide; contacting the alcoholsolution with the first layer; and removing the alcohol solution fromthe first layer and drying the first layer.

The metal halide may include Group II metal (e.g., zinc). The metalhalide may include a fluoride, a chloride, a bromide, an iodide, or acombination thereof. In an embodiment, the metal halide may include azinc chloride.

The preparing of the alcohol solution of the metal halide may includedissolving the aforementioned metal halide in an alcohol solvent (e.g.,C1 to C10 alcohol, for example, methanol, ethanol, propanol,isopropanol, butanol, pentanol, hexanol, heptanol, etc.). A metal halideconcentration in the alcohol solution may be greater than or equal toabout 0.001 grams per liter (g/L), for example, greater than or equal toabout 0.01 g/L, greater than or equal to about 0.1 g/L, greater than orequal to about 1 g/L, greater than or equal to about 10 g/L, greaterthan or equal to about 50 g/L, greater than or equal to about 60 g/L,greater than or equal to about 70 g/L, greater than or equal to about 80g/L, or greater than or equal to about 90 g/L and less than or equal toabout 1000 g/L, for example, less than or equal to about 500 g/L, lessthan or equal to about 400 g/L, less than or equal to about 300 g/L,less than or equal to about 200 g/L, less than or equal to about 100g/L, less than or equal to about 90 g/L, less than or equal to about 80g/L, less than or equal to about 70 g/L, less than or equal to about 60g/L, less than or equal to about 50 g/L, less than or equal to about 40g/L, less than or equal to about 30 g/L, less than or equal to about 20g/L, or less than or equal to about 10 g/L, but is not limited thereto.

Contacting the alcohol solution with the first layer may include addingthe alcohol solution to the first layer in a dropwise fashion, spincoating the alcohol solution after adding the alcohol solution in adropwise fashion, or a combination thereof. The adding in a dropwisefashion (and spin coating) may be performed at least once, for example,at least twice, at least three times, or at least four times.

The removing of the alcohol solution from the first layer may includewashing (e.g., adding the alcohol solvent in a dropwise fashion andoptionally spin coating) the first layer contacting the alcoholsolution, with an alcohol solvent. The washing may be performed at leastonce, for example, at least twice, or at least three times.

The drying of the first layer from which the alcohol solution is removedmay include heating the first layer at a predetermined temperature.

The heating temperature may be greater than or equal to about 30° C.,greater than or equal to about 40° C., greater than or equal to about50° C., greater than or equal to about 60° C., greater than or equal toabout 70° C., greater than or equal to about 80° C., greater than orequal to about 90° C., or greater than or equal to about 100° C. Theheating temperature may be less than or equal to about 200° C., lessthan or equal to about 190° C., less than or equal to about 180° C.,less than or equal to about 170° C., less than or equal to about 160°C., less than or equal to about 150° C., less than or equal to about140° C., less than or equal to about 130° C., less than or equal toabout 120° C., less than or equal to about 110° C., less than or equalto about 100° C., or less than or equal to about 90° C.

The first layer (e.g., the second emission layer in the light emittingdevice) may exhibit changed solubility through a ligand removaltreatment, and accordingly, a quantum dot dispersion may be applied ordeposited on the first layer which is subjected to the ligand removaltreatment, to form the second layer (e.g., the first emission layer inthe light emitting device).

The quantum dot dispersion may include quantum dots having the same (ordifferent) organic ligands as (or from) the ones of the quantum dotsused for the formation of the first layer. The quantum dot dispersionmay be treated with an organic solution including a halogen compound(e.g., a chlorine compound such as a metal chloride) prior to theformation of the second layer. The quantum dot dispersion (e.g., forforming the second layer) may include the halogen treated quantum dotsas described above.

The second layer thus formed may be heat-treated, if desired. Details(e.g., a temperature) of the heat treatment may be the same as describedabove for the heating temperature.

It may not be easy to form a thin film by coating dispersion of quantumdots including the same type of organic ligand multiple times. Withoutwishing to be bound by any theory, it is believed that a solvent in thequantum dot dispersion dissolves a previously coated QD layer. Forexample, the dispersion of quantum dots including the same type oforganic ligand may not be used to form a quantum dot emission filmhaving a desired thickness more than once.

In a method according to an embodiment, sincedissolubility/dispersibility of quantum dots in the layer which issubjected to the ligand removal treatment (e.g., halideexchange-treatment) is changed, the quantum dot layer which is subjectedto halide exchange-treatment is not dissolved by the dispersion ofquantum dots including the same type of organic ligand. In other words,since polarity of the treated first layer is changed, the quantum dotdispersion used for forming the first layer may be coated withoutundesirable dissolution of the first layer which is subjected to theligand removal treatment.

On the formed second layer, a charge auxiliary layer (e.g., an electronauxiliary layer) and an electrode (e.g., cathode) may be optionallyformed. The charge auxiliary layer (e.g., electron auxiliary layer) maybe formed in an appropriate method by considering a material, athickness, and the like of the charge auxiliary layer.

For example, since the formed second layer may not be dissolved in analcohol solvent, when an electron transport layer is formed based on theaforementioned zinc-containing metal oxide nanoparticles, thenanoparticles dispersed in the alcohol solvent may be applied on theaforementioned emission layer.

An embodiment provides an electronic device including the aforementionedlight emitting device. The electronic device may include variouselectronic devices such as display devices or lighting devices.

Hereinafter, embodiments are illustrated in more detail with referenceto examples. However, these examples are exemplary, and the presentscope is not limited thereto.

Analysis Methods 1. Photoluminescence Analysis

Photoluminescence (PL) spectra of the prepared nanocrystal are obtainedusing a Hitachi F-7000 spectrometer at an irradiation wavelength of 372nanometers (nm).

2. Ultraviolet (UV) Spectroscopy

Hitachi U-3310 spectrometer is used to perform a UV spectroscopy andobtain UV-Visible absorption spectra.

3. TEM Analysis

Transmission electron microscope photographs of nanocrystals areobtained using an UT F30 Tecnai electron microscope.

4. X-ray Diffraction (XRD) Analysis

An XRD analysis is performed using a Philips XPert PRO equipment with apower of 3 kilowatts (kW).

5. Electroluminescence Spectroscopy

A current depending on a voltage is measured using a Keithley 2635Bsource meter while applying a voltage and Electroluminescent (EL) lightemitting luminance is measured using a CS2000 spectroscopy.

6. Evaluation of Hole Transport Capability

A hole-only device (HOD) (indium tin oxide(ITO)/poly(3,4-ethylenedioxythiophene)polystyrene sulfonate(PEDOT:PSS)/poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine)(TFB)/quantum dot (QD) emission layer/organicHTL/diipyrazino[2,3f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile(HAT-CN)/Ag) is manufactured in the following method. An ITO patternedsubstrate is subjected to an ultraviolet (UV)-ozone (UVO)-surfacetreatment. A PEDOT:PSS layer is spin-coated to have a thickness of about30 nm and then, heat-treated to remove residual organic materials. A TFBlayer is spin-coated to have a thickness of about 25 nm and then,heat-treated to remove residual organic materials. Quantum dotdispersion is spin-coated to form a 15 to 40 nm-thick emission layer andthen, heat-treated to remove residual organic materials. An upper holetransport layer is formed by sequentially thermally depositing anorganic HTL (e.g., including a compound having a bi-carbazole moiety anda bi-phenyl moiety)/HAT-CN(dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile)layer to be 35 to 40 nm (e.g., 360 Å) thick/10 to 15 nm (e.g., 100 Å)thick. Then, silver (Ag) is thermally deposited under a mask to form anelectrode. The resulting device is sealed with a sealing resin/glass.

Hole transport capability of the manufactured HOD is evaluated bymeasuring a current depending on a voltage by using a Keithley 2635Bsource meter, while the voltage is applied thereto.

6. Infrared Spectroscopy

An infrared spectroscopy is performed using an infrared spectroscopicanalyzer.

7. X-Ray Photoelectron Spectroscopy (XPS) Analysis

An XPS element analysis is performed under the conditions of anacceleration voltage: 0.5 to 15 kiloelectronvolts (keV), 300 watts (W),and a minimum analysis area: 200×200 square micrometers (pmt) usingQuantum 2000 made by Physical Electronics, Inc.

8. Scanning Electron Microscope Energy Dispersive X-Ray Spectroscopy(SEM-EDS) Analysis

A SEM-EDX analysis is performed using a scanning electron microscope.

Synthesis of Quantum Dot Reference Example 1-1: Preparation of BlueLight Emitting Quantum Dot

(1) Selenium (Se) and tellurium (Te) are dispersed in trioctylphosphine(TOP) to obtain a Se/TOP stock solution and Te/TOP stock solution,respectively. 0.125 millimoles (mmol) of zinc acetate is added witholeic acid (OA) to a reactor including trioctylamine and vacuum-treatedat 120° C. After 1 hour, an atmosphere in the reactor is replaced withnitrogen.

Subsequently, the reactor is heated up to 300° C., the prepared Se/TOPstock solution and Te/TOP stock solution are rapidly injected thereintoin a Te/Se ratio of 1/25. When the reaction is complete, acetone isadded to the reaction solution that is rapidly cooled into roomtemperature, and a precipitate obtained by centrifugation is dispersedin toluene to obtain a ZnTeSe core.

(2) 1.8 mmoL (0.336 grams (g)) of zinc acetate is added with oleic acidto a flask including trioctylamine and vacuum-treated at 120° C. for 10minutes. The atmosphere in the flask is substituted with nitrogen (N₂)and a temperature is increased up to 180° C. The ZnTeSe core obtainedabove is added and Se/TOP stock solution and S/TOP stock solutionprepared by dispersing sulfur (S) in trioctylphosphine are injected. Thereaction temperature is set to be about 280° C. After the reaction iscomplete, the reactor is cooled and the prepared nanocrystal iscentrifuged with ethanol and is dispersed in an organic solvent (e.g.,toluene or octane) to obtain a dispersion of ZnTeSe/ZnSeS core/shellquantum dots.

Reference Example 1-2: Preparation of Red Light Emitting Quantum Dot

(1) 0.2 mmol of indium acetate is dissolved with palmitic acid in a 300milliliters (mL) reaction flask including 1-octadecene and heated at120° C. under vacuum. After 1 hour, atmosphere in the reactor isreplaced with nitrogen. After heating at 280° C., a mixture oftris(trimethylsilyl)phosphine (TMS₃P) trioctylphosphine are rapidlyinjected and reacted for 30 minutes. Acetone is added to the reactionsolution which has been rapidly cooled at room temperature andcentrifuged to provide a precipitate (i.e., InP core), and the obtainedprecipitate is dispersed in toluene.

A Se powder and a S powder are dissolved in TOP to prepare a Se/TOPstock solution and an S/TOP stock solution, respectively.

Zinc acetate and oleic acid are dissolved in trioctylamine in a 300 mLreaction flask and vacuum-treated at 120° C. for 10 minutes. Theatmosphere in the flask is substituted with nitrogen (N₂) and thenheated at 180° C.

The obtained InP core is added and a predetermined amount of the Se/TOPstock solution and a predetermined amount of the S/TOP stock solutionwere added, and a reaction is performed at reaction temperature of 280°C. for 60 minutes.

An excess amount of ethanol is added into the reaction mixture includingthe semiconductor nanocrystal and centrifuged. After the centrifugation,a supernatant is discarded, the precipitate is dried and then dispersedin chloroform or toluene to provide a InP/ZnSeS core/shell quantum dotsolution (hereinafter, QD solution). The obtained QD solution ismeasured for a UV-vis absorption spectrum. The obtained quantum dotsemit red light.

Synthesis of Metal Oxide Nanoparticles Reference Example 2: Synthesis ofZn Metal Oxide Nanoparticles

Zinc acetate dihydrate and magnesium acetate tetrahydrate are added todimethylsulfoxide in a reactor so that a mole ratio of the followingchemical formula is provided, and the reactor is heated at 60° C. in theair. Subsequently, an ethanol solution of tetramethyl ammonium hydroxidepentahydrate is added in a dropwise fashion thereto at a rate of 3milliliters per minute (mL/min). The obtained mixture is stirred for onehour, and Zn_(x)Mg_(1-x)O nanoparticles produced therein are centrifugedand dispersed in ethanol to obtain Zn_(x)Mg_(1-x)O (x=0.85)nanoparticles.

The obtained nanoparticles are performed with an X-ray diffractionanalysis to confirm that ZnO crystals are formed. A transmissionelectron microscopic analysis is performed for the obtainednanoparticles, and the results shows that the particles have an averagesize of about 3 nm.

The obtained nanoparticles are measured for their UV-Vis absorptionspectrum by using UV-Vis Spectrophotometer (UV-2600, SHIMADZU), and anenergy bandgap of the nanoparticles is obtained from a UV band edgetangent line. The results show that the synthesized Zn_(x)Mg_(1-x)O hasan energy bandgap of about 3.52 electronvolts (eV) to 3.70 eV.

Spin-Dry Treatment Reference Example 3-1

A quantum dot emission layer is formed by spin-coating an octanedispersion of the core/shell quantum dots according to Reference Example1-1 on a silicon substrate and heat-treating at 80° C. for 30 minutes.An infrared ray spectroscopy regarding the obtained quantum dot emissionfilm is performed to measure COO⁻ peak intensity relative to a Si peak,and the result is shown in Table 1 and FIG. 4.

An X-ray photoelectron spectroscopy regarding the quantum dot emissionfilm is performed, and the result is shown in Table 2.

A SEM-EDX analysis regarding the spin-dried quantum dot emission film isperformed, and the result is shown in Table 3.

Reference Example 3-2

A solution prepared by dissolving zinc chloride in ethanol(concentration: 0.1 grams per milliliter (g/mL)) is added in a dropwisefashion on the quantum dot emission layer formed in Reference Example3-1, allowed to stand for one minute, partially removed with aspin-coater, and three times washed with ethanol. The washed first layeris dried on an 80° C. hot plate for 20 minutes. An infrared spectroscopyregarding the obtained (hereinafter, spin-dried) quantum dot emissionfilm is performed to measure a COO— peak intensity relative to an Sipeak, and the result is shown in Table 1 and FIG. 4.

An X-ray diffraction analysis of the spin-dried quantum dot emissionfilm is performed, and the result is shown in Table 2. The X-raydiffraction analysis result shows a defect, which is regarded as ZnCl₂,on the spin-dried layer.

A SEM-EDX analysis regarding the spin-dried quantum dot emission film isperformed, and the result is shown in Table 3.

Reference Example 4-1

A quantum dot emission layer is formed by spin-coating an octanedispersion of the core/shell quantum dots according to Reference Example1-2 on a silicon substrate and heat-treating at 80° C. for 30 minutes.An infrared spectroscopy regarding the quantum dot emission film isperformed to measure COO— peak intensity relative to an Si peak, and theresult is shown in Table 1 and FIG. 5.

Reference Example 4-2

A solution prepared by dissolving zinc chloride in ethanol(concentration: 0.1 g/mL) is added in a dropwise fashion on the quantumdot emission layer formed in Reference Example 4-1, allowed to stand for1 minute, partially removed with a spin-coater, and three times washedwith ethanol (EtOH). The washed first layer is dried on an 80° C. hotplate for 20 minutes. An infrared spectroscopy regarding the obtained(hereinafter, spin-dried) quantum dot emission film is performed tomeasure COO— peak intensity relative to an Si peak, and the results areshown in Table 1 and FIG. 5.

TABLE 1 COO/Si peak Oleic Acid (OA) intensity decrease Reference Example3-1 1.13 reference Reference Example 3-2 0.78 31.1% Reference Example4-1 1.50 reference Reference Example 4-2 0.85 43.2%

Referring to the results of Table 1 and FIGS. 4 and 5, amounts of theorganic ligands bound to quantum dots during the synthesis are greatlydecreased through the spin-dry treatment, and accordingly, chloride(Cl—) instead of COO— may be bound to the quantum dots included in thequantum dot thin film.

TABLE 2 C1s Zn2p3 Reference Example 3-1 55.92 20.88 (OA-QD) ReferenceExample 3-2 38.57 32.29 ZnCl₂/EtOH = 100 milligrams per milliliter(mg/ml) (Spin-dry OA-QD)

Referring to Table 2, an amount of carbon relative to that of zinc isgreatly reduced through the spin dry treatment (from 55.92:20.88 to38.57:32.29).

TABLE 3 Reference Example 3-1: Reference Example 3-2: Atomic ratio OA-QDSpin-dry OA-QD Cl:Zn 0.0:1 0.10:1

Referring to Table 3, an amount of chlorine relative to that of zinc isincreased in a significant level (from 0.0:1 to 0.10:1).

Preparation of Halogen Treated Quantum Dots Reference Example 4-3

Quantum dots obtained by Reference Example 1-1 are dispersed in tolueneto obtain a quantum dot organic dispersion. Zinc chloride is dissolvedin ethanol to obtain a zinc chloride solution having a concentration of10 wt %. 0.01 mL of the obtained zinc chloride solution is added to theprepared quantum dot organic dispersion and then, stirred at 60° C. for30 minutes to perform a surface exchange reaction. After the reaction,ethanol is added thereto to induce a precipitation, and the quantum dotsare recovered through centrifugation. With respect to the recoveredquantum dots, the surface exchange reaction is repeated to obtainhalogen-treated quantum dots.

Hole Transport Capability Evaluation of Spin-Dried Film ReferenceExample 5-1

Hole transport capability of the emission layer including quantum dotsaccording to Reference Example 1-1 is evaluated before and after thespin-dry treatment. The hole transport capability is evaluated bymanufacturing a hole only device (ITO/PEDOT:PSS (30 nm)/TFB (25 nm)/QDemission layer (20-30 nm)/organic HTL (36 nm)/HAT-CN (10 nm)/Ag (100nm)) respectively including a non-spin-dried quantum dot emission layerand the spin-dried quantum dot emission layer according to ReferenceExample 3-2. As a result, hole transport capability (log scale) of afirst layer after the treatment is 148 milliamperes per squarecentimeter (mA/cm²) at 8 volts (V)), which is about 9867 times higherthan hole transport capability (log scale, 0.015 mA/cm² at 8 V) of thefirst layer before the treatment.

Reference Example 5-2

Hole transport capability of the layer (SD treated BQD layer) includingquantum dots according to Reference Example 1-1 and being spin-drytreated and the layer (C1-treated BQD layer) including quantum dotsaccording to Reference Example 4-3 is measured as in the same manner setforth in Reference Example 5-1. The results confirm that the HTcapability of the SD treated BQD layer is 2.18 times higher than that ofthe C1-treated BQD layer.

Manufacture of Light Emitting Device Example 1-1

A device having a laminate structure of ITO/PEDOT:PSS (30 nm)/TFB (25nm)/Blue OA less (spin-dry) QD (20 nm)/Blue OA QD (12 nm)/ZnMgO (20nm)/Al (100 nm) is manufactured as follows.

1. An ITO-deposited glass substrate is surface-treated with UV-ozone for15 minutes and then spin-coated with a PEDOT:PSS solution (H.C. Starks)and heated at 150° C. for 10 minutes under an air atmosphere, and thenis heat-treated again at 150° C. for 10 minutes under an N₂ atmosphereto provide a hole injection layer having a thickness of 30 nm.Subsequently, apoly[(9,9-dioctylfluorenyl-2,7-diyl)-co(N-(4-butylphenyl)diphenylamine]solution (TFB) (Sumitomo) is spin-coated on the hole injection layer andheat-treated at 150° C. for 30 minute to form a hole transport layer.

2. On the obtained hole transport layer, core-shell quantum dotsprepared by Reference Example 1-1 are spin-coated and heat-treated at80° C. for 30 minutes to form a quantum dot layer. Zinc chloride isdissolved in ethanol to prepare a treating solution (concentration: 0.1g/mL). The treating solution is added in a dropwise fashion on theformed quantum dot layer, kept it as it is, removed, and washed withethanol several times. The washed quantum dot layer is dried on a 80° C.hot plate to obtain a second emission layer as ligand-removal treated.

3. The dispersion of the core/shell quantum dots according to ReferenceExample 1-1 is spin-coated on the second emission layer asligand-removal treated and then is heat-treated at 80° C. for 30 minutesto form a first emission layer.

4. A solution of the ZnMgO nanoparticles prepared in Reference Example 2(solvent: ethanol, optical density: 0.5 a.u) is prepared. The solutionis spin-coated on the first emission layer and heat-treated at 80° C.for 30 minutes to form an electron auxiliary layer. On a portion of thesurface of the electron auxiliary layer, aluminum (Al) isvacuum-deposited to form a second electrode to manufacture a lightemitting device shown in FIG. 1.

Electroluminescent properties of the obtained quantum dot light emittingdevice are evaluated using a Keithley 2200 source measuring device and aMinolta CS2000 spectroradiometer (current-voltage-luminance measurementequipment). A current depending upon a voltage applied to the device,luminance, and electroluminescence (EL) are measured by thecurrent-voltage-luminance measurement equipment, and thereby externalquantum efficiency is calculated. The results are shown in Table 4, andFIGS. 6 and 7.

Comparative Example 1

An electroluminescent device (layer structure: ITO/PEDOT:PSS (30 nm)/TFB(25 nm)/Blue OA less (spin-dry) QD (20 nm)/ZnMgO (20 nm)/Al (100 nm)) ismanufactured according to the same method as Example 1 except for notforming the first emission layer.

The obtained quantum dot light emitting device is evaluated forelectroluminescent properties using a Keithley 2200 source measuringdevice and a Minolta CS2000 spectroradiometer (current-voltage-luminancemeasurement equipment). A current depending upon a voltage applied tothe device, luminance, and electroluminescence (EL) are measured by thecurrent-voltage-luminance measurement equipment, and thereby externalquantum efficiency is calculated. The results are shown in Table 4,FIGS. 6 and 7.

Comparative Example 2

A light emitting device (layer structure: ITO/PEDOT:PSS (30 nm)/TFB (25nm)/Blue OA QD (20 nm)/ZnMgO (20 nm)/Al (100 nm)) is manufacturedaccording to the same method as Example 1 except for not performing theligand removal (spin dry) treatment during the forming the secondemission layer and not forming the first emission layer.

The obtained quantum dot light emitting device is evaluated forelectroluminescent properties using a Keithley 2200 source measuringdevice and a Minolta CS2000 spectroradiometer (current-voltage-luminancemeasurement equipment). A current depending upon a voltage applied tothe device, luminance, and electroluminescence (EL) are measured by thecurrent-voltage-luminance measurement equipment, and thereby externalquantum efficiency is calculated. The results are shown in Table 4 andFIGS. 6 and 7.

TABLE 4 Cd/m² @ 5 milli- Lamda Max. EQE @ Max. amperes max. MaxDescriptions EQE 100 nit Cd/A (mA) (nm) Lum Comparative Example 3.2 2.21.6  44 454 3740 2 (Blue (B) OA single emission layer) ComparativeExample 2.7 2.6 2.4 122 461 4940 1 (B spin-dry single emission layer)Example 1-1 (B spin- 6.5 5.3 4.7 228 457 5070 dry/OA double emissionlayer) * Max. EQE: maximum external quantum efficiency * EQE @ 100 nit:external quantum efficiency at luminance of 100 nit (candelas per squaremeter (cd/m²)) * Max. Cd/A (Candelas per ampere): maximum currentefficiency * λmax: maximum photoluminescence wavelength * Max Lum:maximum luminance (cd/m²)

Referring to the results of Table 4 and FIGS. 6 and 7, the device ofExample 1 may show improved efficiency and improved luminance but areduced leakage current compared with the devices of ComparativeExamples 1 and 2.

Example 1-2

A light emitting device (layer structure: ITO/PEDOT:PSS (30 nm)/TFB (25nm)/Blue OA less (spin-dry) QD (20 nm)/halogen treated QD (12 nm)/ZnMgO(20 nm)/Al (100 nm)) is manufactured according to the same method asExample 1 except for the following:

A dispersion of the halogen treated quantum dots prepared in ReferenceExample 4-3 is spin-coated on the second emission layer asligand-removal treated and then heat-treated at 80° C. for 30 minutes toform a first emission layer.

The obtained quantum dot light emitting device is evaluated forelectroluminescent properties using a Keithley 2200 source measuringdevice and a Minolta CS2000 spectroradiometer (current-voltage-luminancemeasurement equipment). A current depending upon a voltage applied tothe device, luminance, and electroluminescence (EL) are measured by thecurrent-voltage-luminance measurement equipment, and thereby externalquantum efficiency is calculated.

The results confirm that in comparison with the device of ComparativeExample 1 (B spin dry single layer), the max EQE and the max luminanceof the device of Example 1-2 increase by 3.7 times and 3.6 times,respectively.

Experimental Example 1

A quantum dot emission layer (a thickness of 25 nm) is formed on aSi-wafer in the same manner as in Reference Example 4-2 and a Pt overcoat is formed. Using a TEM-EDX equipment manufactured by Tecnai Co.,Ltd (Titan G2) having EDS of SuperX, a Line scan analysis is made andthe results are shown in FIG. 8.

The results show that the spin-dry treated quantum dot emission layerincludes a noticeable amount of chlorine.

Experimental Example 2

Two quantum dot emission layers (having a thickness of 40 nm) are formedon a Si-wafer in the same manner as in Reference Example 4-1 (Blue OA)and Reference Example 4-2 (Blue Spin dry), respectively. While etchingeach of the quantum dot emission layers thus formed via plasma etching,an XPS analysis is made and the results are shown in Table 5.

TABLE 5 etching Blue OA Blue Spin-dry time (min) C12p** Zn2p3 C12p Zn2p30 0.0 26.1 1.5 25.7 1 0.0 54.7 1.1 54.1 2 0.0 55.7 1.1 55.1 3 0.0 57.60.9 57.1 4 0.0 58.2 0.5 57.9 5 0.0 61.0 1.1 60.3

The results confirm that the quantum dot emission layer formed by themethod of Reference Example 4-2 includes a substantial amount of thechlorine.

Experimental Example 3: Lifetime of the Device

For each of the devices of Examples 1-1 and 1-2 and the devices ofComparative Examples 1 and 2, the lifetime of the device (T50) ismeasured:

T(50): When being operated at 325 nit, times that is taken for 50%decrease of the luminance of the device with respect to its initialvalue (100%).

The results confirm that the device of Comparative Example 1 (B spin-drysingle layer) shows the T50 about 20% less than that of the device ofComparative Example 2 (B OA single layer) while the devices of Examples1-1 an 1-2 show the T50 values about 1.9 times and 7 times greater thanthat of the device of Comparative Example 2.

While this disclosure has been described in connection with what ispresently considered to be practical example embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

DESCRIPTION OF SYMBOLS

-   -   10: light emitting device    -   11: first electrode    -   12: hole auxiliary layer    -   13: emission layer    -   14: electron auxiliary layer    -   15: second electrode

What is claimed is:
 1. A light emitting device, comprising a firstelectrode and a second electrode facing each other, an emission layerdisposed between the first electrode and the second electrode, theemission layer comprising quantum dots and a first organic ligand, andan electron auxiliary layer disposed between the emission layer and thesecond electrode, wherein the emission layer comprises a first surfacefacing the electron auxiliary layer and an opposite second surfacefacing away from the electron auxiliary layer, and in the emissionlayer, an amount of the first organic ligand in a portion adjacent tothe first surface is greater than an amount of the first organic ligandin a portion adjacent to the second surface.
 2. The light emittingdevice of claim 1, wherein the amount of the first organic ligand in theportion adjacent to the first surface is at least about 20% larger thanthe amount of the first organic ligand in the portion adjacent to thesecond surface.
 3. The light emitting device of claim 1, wherein theelectron auxiliary layer is an electron auxiliary layer comprisingnanoparticles comprising zinc metal oxide.
 4. The light emitting deviceof claim 3, wherein the zinc metal oxide is represented by ChemicalFormula 1:Zn_(1-x)M_(x)O  Chemical Formula 1 wherein, in the Chemical Formula 1, Mis Mg, Ca, Zr, W, Li, Ti, Y, Al, or a combination thereof, and 0≤x≤0.5.5. The light emitting device of claim 3, wherein the metal oxidecomprises zinc oxide, zinc magnesium oxide, or a combination thereof. 6.The light emitting device of claim 3, wherein an average particle sizeof the nanoparticles is greater than or equal to about 1 nanometer andless than or equal to about 10 nanometers.
 7. The light emitting deviceof claim 1, wherein a work function of the first electrode is greaterthan a work function of the second electrode.
 8. The light emittingdevice of claim 1, wherein the first electrode comprises indium tinoxide, or wherein the second electrode comprises a conductive metal. 9.The light emitting device of claim 1, wherein the quantum dots do notcomprise cadmium.
 10. The light emitting device of claim 1, wherein thequantum dots comprise a first compound comprising indium and phosphorus,a second compound comprising chalcogen element and zinc, or acombination thereof.
 11. The light emitting device of claim 1, whereinthe portion adjacent to the first surface comprises the first organicligand and the portion adjacent to the second surface comprises thefirst organic ligand or a second organic ligand.
 12. The light emittingdevice of claim 1, wherein the quantum dots are configured to emit lighthaving a same color.
 13. The light emitting device of claim 1, whereinin the emission layer, the portion adjacent to the second surfacefurther comprises a halogen.
 14. The light emitting device of claim 1,wherein the halogen comprises fluorine, chlorine, bromine, iodine, or acombination thereof.
 15. The light emitting device of claim 1, whereinthe first organic ligand comprises RCOOH, RNH₂, R₂NH, R₃N, RSH, R₃PO,R₃P, ROH, RCOOR, RPO(OH)₂, RHPOOH, RHPOOH, or a combination thereof,wherein, R is independently a substituted or unsubstituted C3 to C40aliphatic hydrocarbon group, a substituted or unsubstituted C6 to C40aromatic hydrocarbon group, or a combination thereof.
 16. The lightemitting device of claim 1, wherein a thickness of the emission layer isgreater than or equal to about 2 nanometers and less than or equal toabout 100 nanometers.
 17. The light emitting device of claim 1, whereinthe emission layer comprises a first emission layer comprising the firstsurface and a second emission layer comprising the second surface, andoptionally wherein the first emission layer comprises the first organicligand and the second emission layer comprises the first organic ligand.18. The light emitting device of claim 17, wherein a thickness of thefirst emission layer is greater than or equal to about 3 nanometers andless than or equal to about 50 nanometers.
 19. The light emitting deviceof claim 17, wherein a thickness of the second emission layer is greaterthan or equal to about 3 nanometers and less than or equal to about 50nanometers.
 20. The light emitting device of claim 17, wherein thesecond emission layer further comprises a halogen comprising chlorine,fluorine, bromine, iodine, or a combination thereof.
 21. A displaydevice comprising the light emitting device of claim 1.